On my game, I have an error every time my game goes from slot 0 to slot 1 when it is used right after another program is used involving slot 1 (Or right when it starts up or when slot one is loaded to something else), then it works after a second time. Is there a way to fix this?

I think no.

It says 'undefined label'. Are you trying to use a label that doesn't exist yet in the program? Trying to use commands like GOTO, GOSUB, and RESTORE with a label that does not yet exist will result in said error. You could try closing the program in Slot 1 by using the NEW command, which will empty that slot, but I believe it is DIRECT mode only.

Did you use the USE command? Did you specify the GOTO label with a "1:@labelname"? Both of these are required or the labels of another slot will be inaccessible.