Trying to Charge Honda Fit 2015, Car's Headlights Flash Once Charging Starts

I’m not a car person at all, so thanks in advance for any help!

My wife’s 2015 Honda Fit hasn’t been used for months (thanks to Corona). I’m trying to use a Schumacher SC1307 6/12V Automatic Battery Charger and 30/85A Engine Starter, to charge the battery using a wall current, to get it running again.

I was able to hook everything up, but when I turned the charger on, the car started blinking the headlights every second. Since I imagine it can’t charge well while doing that, I turned it off … but now I’m unsure how to charge the battery.

I saw something online that suggested I disconnect the battery first, but:

A) other sites don’t mention anything about Honda Fits specifically needing to disconnect the battery to get a jump/charge

B) I don’t know how to disconnect the battery

So my question is, do I need to disconnect the battery? If not, how can I charge the car without disconnecting it, and if I do need to disconnect it … how do I do that on a Honda Fit?