2018 Nissan Sentra SR Brake stuck, wouldn't start

My car has been fine for 4 years, got CPO had 5 k miles, now
77k. This evening I went to start the car, got someone nearby
with a battery pack and tester, they said it was good, but they
thought it wouldn't start due to a brake sensor. I called AAA for
a tow, and the contractor driver came and started to charge
the battery, he then managed to get the brake to unlock and
the car started. He also mentioned the battery is probably bad
and he said that I would have to hold the key fob right against
the pushbutton start until I get the battery replaced. I am
wondering what the actual problem is, the battery (possibly
alternator too )a brake sensor or switch, or something else?
Any advice or recommendations? I love this car and except for
some typical routine maintenance and the fact it doesn't like
going up hills it's been fine to me (knock on wood).