2000 SL2 will not start

This morning got up and started the car to go run some errans. Car started and immediately died. Tried to start back and everything is dead. No lights, horn, clock, radio, nothing. The battery is less than a year old. I don't know if this has anything to do with it but recently I noticed my dashboard lights with a slight flicker every now and then. I had the alternator tested after that happened and it was fine. This is my only car right now. Anyone have any ideas?
Thanks

How did you do an alternator test with the car off?

What is the battery voltage at? What is it when you try to start the car?

I tested the alternator back when the dash lights were flickering. The battery is at 12.4

What is the voltage when you try to start the car?

Remove your battery connections and make sure they are clean