[SilentSoul]

You can simply use something like that.

pawn Код:

CMD:setlevel(playerid,params[])
{
    SetPlayerScore(playerid,GetPlayerScore(playerid)+1); //getting the current player score and increase the value +1 , no need for all codes above.
    SendClientMessage(playerid, COLOR_RED, "Your level has been set!");
    return 1;
}

Sources :
https://sampwiki.blast.hk/wiki/SetPlayerScore
https://sampwiki.blast.hk/wiki/GetPlayerScore