(1119) : error 002: only a single statement (or expression) can follow each "case"
(1119) : error 029: invalid expression, assumed zero
(1119) : warning 215: expression has no effect
(1119) : error 017: undefined symbol "i"
(1119) : fatal error 107: too many error messages on one line

if(dialogid == DIALOG_INV) {
        if(response) {
            new msg[100], Float:X,Float:Y,Float:Z;
            switch(listitem) {
                    for(new i=0; i<sizeof(listitem); i++) { // <--------- THE LINE
                
                    case i: {
                        if(PlayerInv[i][playerid] == 0) return SendClientMessage(playerid,CLR_GREY,"This slot is empty.");
                        else if(PlayerInv[i][playerid] == 1) {
                            GivePlayerWeapon(playerid,4,1);
                            PlayerInv[i][playerid] = 0;
                        }
                    }
                }
            }
        }
    }