[[XST]O_x]

You're checking if the player is an admin, and then telling him that he can't use that command. (?)
Hopelessness indeed, lol.

pawn Код:

if(!IsPlayerAdmin(playerid)) return SendClientMessage(playerid, 0xFF4646FF, "You are not authorized to use that command !"); 

else
{
  new otherId;
  if(sscanf(params, "u", otherId)) return SendClientMessage(playerid, 0xFF4646FF, "USAGE: /slap [ID/PartOfname]");
   if(otherId != INVALID_PLAYER_ID)
   {
       new Float:slx, Float:sly, Float:slz;
       GetPlayerPos(otherId, slx, sly, slz);
       SetPlayerPos(otherId, slx, sly, slz+5);
       PlayerPlaySound(otherId, 1130, slx, sly, slz+5);
       SendClientMessage(playerid, 0xFFFFFFAA, "(( You have been slapped by admin ! ))");
}

Also, why formatting a string without any contents? Only plain text? You don't need to format for that, useless variable.