I've got a 2015 premium bmx. When I start to pedal, the left crank kinda slips, if that doesn't make sense, let me siimplify it. When I pedal, it's like it doesn't grab the chain or something, like it pedals and moves the chain but the wheel doesn't spin, only at first tho then it rides fine. But only when I try to pedal from the left side. I also have a freecoaster, maybe that's why but I don't know
Your freewheel is failing - the pawls are not grabbing immediately which allows the chain to move without the rear wheel being powered.
This will not resolve itself over time, it will get worse.
Your two solutions are
Temporary) try washing the freewheel with solvent to get the gunk out. Then let it dry and relube with light machine oil.
Longterm) Replace the freewheel mechanism with a new one. You will probably need a new chain at this time too.