[Jstylezzz]

Try

pawn Код:

CMD:armour(playerid,params[])
{
    if(!IsPlayerAdmin(playerid)) return 0; //If the player is not admin show 'unknown command'
    SendClientMessage(playerid,-1,"You've sucessfully used armour!");
    SetPlayerArmour(playerid,100.0);
    return 1;
}

EDIT: Stinged beat me to it :P
EDIT2: The reason that I used return 0 after IsPlayerAdmin, and did not use SendClientMessage, is simply because the normal players don't have to know that the command exists.