[Toroi]

https://sampwiki.blast.hk/wiki/Format

The output of a format always has to be a string value, however you're trying to fit it into an integer

Код:

new exp = GetPlayerScore(playerid); 
format(exp,sizeof(exp),"Exp:%i/%i",GetPlayerScore(playerid),JumlahExp[playerid]);

You almost have your code right. Convert the exp integer to a string and remove that value assignation to the GetPlayerScore function. In the following line, instead of exp inside the SetPlayerScore function you can use the GetPlayerScore function itself plus one.

Try it and come back if you need more help.