Why is Arduino Pro Mini not powering this LED?

First off, I’m very new to Arduinos and electronics in general, so feel free to explain like I’m 5.

As a bit of background to this question, the project is simply using a Pro Mini to run a servo, with a button to turn it on and off, and an led to indicate when it’s on. To make sure I was getting the button right, I built the circuit on a breadboard with just the button and LED. The button was supposed to switch the LED on and off. It did not.

I removed the button from the equation and just tried to power the LED with the Mini. No go.

It’s a blue LED, so the resistor (100) was chosen assuming a forward voltage of 3.3V and current of .02A. I’ve tested the resistor and LED combo with JUST the battery pack, and it lights up bright like it should.

I have a 9V battery running into RAW (I’ve also tried a 6V pack). I had pin 2 as an output with digitalWrite(HIGH), running into the resistor, then into the LED (+, long end), and then back to a ground pin. I’ve tried other ground pins on the board to make sure it’s not that. I’ve switched pin 2 to analogWrite(255), still nothing. I have tried connecting it to VCC instead. Nothing.

However, writing HIGH to the onboard LED works fine, so I can see that the sketch is indeed running.

I’ve verified the LED polarity. I have tried a different Pro Mini. Nothing. It’s a Pro Mini 328P according to the stamp on the back, which Google says runs at 5V and 16 MHz.

Please tell me what stupid mistake I am making so I can stop wanting to rip my hair out. Thank you.