[Beckett]

pawn Код:

if(IsPlayerInAnyVehicle(playerid))
        {
            SendClientMessageEx(playerid, COLOR_GREY, "You must be on foot to use this command.");
            return 1;
        }

You are checking if the player is in a vehicle, if yes then you send this message..

pawn Код:

if(!IsPlayerInAnyVehicle(playerid))
        {
            SendClientMessageEx(playerid, COLOR_GREY, "You must be on foot to use this command.");
            return 1;
        }

Opposite.