What should i do about my boss?

Hey everyone!

Im 20, started working my job at a fast food place a little over a year ago.

About three weeks ago, i told my GM i needed the fourth off. I’ll be out of state at this time, there’s no way i could make the shift. He said that was fine and plenty of notice. But he did inform me to put it down on the “find a cover sheet” just for formality. So i did. And, for the record, we only need two weeks notice. I gave three trying to be cautious

I texted the group chat, asked coworkers personally, and no one could. He said it was fine still. Yesterday, he asked if i was excited to work on the fourth. I said “okay, huh? I thought i wasn’t working. Im going to be out of state”

And then he said.

“Nope, you’re working. It’s your job to cover it, not mine.” I explained that i tried and no one would take it, also said that i hadn’t seen my family in over a year now and I couldn’t reschedule it. He told me essentially if im not there he will fire me. Im really upset. My moms been looking forward to this all summer. I dunno what to do