What is a word/expression for useless advice?

If the advice is useless because it's bad or misleading, that's called a "bum steer."

There's a vulgarity for that, but the colloquialism my mom uses for useless advice is "bunch of hooey," like if I knew she'd gone to seek someone's advice and asked her how it went, she might say, "All he gave me was a bunch of hooey," meaning all he gave her was a bunch of worthless advice.

"It's not worth the paper it's written on."

When a suggestion or advice is useless or not important, even in a case where it's not actually on paper, you can say it's not worth the paper it's written on. This usually involves things that are true but simply not helpful or worthwhile. An example from TFD:

Oh, that memo is not worth the paper it's written on. You know the boss is going to completely change her mind about it in a few days anyway!

Definitions include useless, unimportant, insignificant.

There is a business concept of 'true but useless', more commonly applied to information rather than advice, but it may capture some of what you are looking for. The phrase 'technically correct' also conveys the same idea (the inclusion of 'technically' means that 'useless' can be left out and would be understood).

From here:

I have sat in too many meetings where people have established facts which everyone in the room agrees with. The problem is often that these facts are completely irrelevant to any action that can be taken. “If we had more time we could do this” is said when no time is available. If you can’t do anything about something why focus on it?

It also calls to mind this old joke:

A helicopter with a pilot and a single passenger was flying around above Seattle when a malfunction disabled all of the aircraft's navigation and communications equipment. Due to the darkness and haze, the pilot could not determine the helicopter's position and course to get back to the airport.

The pilot saw a tall building with lights on and flew toward it, the pilot had the passenger draw a handwritten sign reading, "WHERE AM I?", and hold it up for the building's occupants to see.

People in the building quickly responded to the aircraft, drew a large sign, and held it in a building window. Their sign said, "YOU ARE IN A HELICOPTER."

The pilot smiled, waved, looked at his map, determined the course to steer to SEATAC airport, and landed safely. After they were on the ground, the passenger asked the pilot how the "YOU ARE IN A HELICOPTER" sign helped determine their position.

The pilot responded, "I knew that had to be the Microsoft support building, they gave me a technically correct but entirely useless answer."

One colloqual answer to that type of advice could be "Tell me something I don't know?".

And the equivalent to your Czech Count in popular english expressions would be "backseat driver" or "armchair coach". So the advice could be characterized as "backseat driver advice", or "armchair coach advice".

You lose the "haughty" aspect of the Czech expression, but it's the best I could come up with.

The phrase "you're not wrong" (or "they're not wrong") seems appropriate. It is used in reference to any statement that is true but not applicable, and/or incomplete.

Compared to most of the other answers, which have been in the vernacular for decades or more, I believe "you're not wrong" is a relatively recent/new phrase.

In mathematics and logic, a Vacuous Truth or "vacuously true statement" is a statement that is only true because the antecedent cannot be satisfied.

Example from Wikipedia:

For example, the statement "all cell phones in the room are turned off" will be true even if there are no cell phones in the room. In this case, the statement "all cell phones in the room are turned on" would also be vacuously true, as would the conjunction of the two: "all cell phones in the room are turned on and turned off". For that reason, it is sometimes said that a statement is vacuously true only because it does not really say anything.

As a side note, using vacuously true statements with insufficient context may invite assumptions about the antecedent, or imply that . As noted in comments by @Gregory Currie, if I heard the statement "All cell phones are turned on", my first assumption is that there is at least one, and probably more than one cell phone in the room. This side effect is sometimes used on purpose, to mislead the audience "without technically lying". For example, a teenager may say to their mom, "No, I promise I didn't come home too late last night!", without mentioning that they actually didn't come home last night at all. In this way, they avoid the sin/crime of lying to their parents, but avoid revealing the incriminating truth.

This effect is similar to Lying by Omission, in that it's commonly thought of as a type of deception, but somehow not as bad as telling an outright lie. As noted by @dbmag9, these are ways of slightly violating one or two of the Maxims of the Cooperative Principle, just not the specific Maxim that requires not telling lies.

This phrase is, unfortunately, not that common among people who didn't study math or logic. So perhaps this Answer itself is vacuously true, in the sense that the phrase technically fits, but likely doesn't solve your need.

You could get close to your sentiment by thanking Captain Obvious. The emphasis is less on how useless the information is and more on how self-evident, but there's often overlap. Your example of "Well if you didn't get a loan in the first place, you wouldn't be in this situation today..." could easily be replied to with a nice sarcastic "Thank you, Captain Obvious". If he's not around, you could go with Captain Hindsight instead, for when the advice is only obvious now and might not have been in the past. Either way, it's technically correct but useless advice, so could work for your scenario.

"hindsight is 2020 vision" is fairly close. e.g. the information of the future is not known in the past and it's easy to know what you should have done now.

Or "teaching someone to suck an egg" https://en.wikipedia.org/wiki/Teaching_grandmother_to_suck_eggs is an English language expression for useless/unwated advice.

An equivalent American English idiom would be smart ideas, although it doesn't really fit in with your example

A: Man these loans are killing me. I can barely keep up the payments on time. What should I do?

B: Well if you didn't get a loan in the first place, you wouldn't be in this situation today...

A: Please stop with your smart ideas

A better example for this term would be

A: Man these loans are killing me. I can barely keep up the payments on time. What should I do?

B: Well you could rob a bank and pay them all off at once...

A: Please stop with your smart ideas

There is always the phrase you already constructed to ask the question:

useless advice

A: Man these loans are killing me. I can barely keep up the payments on time. What should I do?
B: Well if you didn't get a loan in the first place, you wouldn't be in this situation today...
A: Please stop with your useless advice.

This is distinct from bad advice, which makes things worse. Useless advice isn't inherently bad, but it doesn't make things better either.

You could say "that isn't helpful", but it doesn't directly translate to a phrase you can fit into your example sentence.

Unhelpful advice would work, but I haven't heard it a lot. Google ngrams shows it's relatively common though.

If it's meant to be aggressive, if person A is annoyed by B and wanting to express that, then it would be extremely idiomatic to use sarcasm instead.

A: Man these loans are killing me. I can barely keep up the payments on time. What should I do?
B: Well if you didn't get a loan in the first place, you wouldn't be in this situation today...
A: Wow! Thanks for your outstandingly useful and helpful advice, you're truly a genius!

The more excessive the praise, the ruder the retort.