[CentyPoo]

pawn Код:

SendClientMessage(killerid, COLOR_YELLOW, "Congratulations you killed %s !", playerid);

Your (playerid) was in the wrong place, when you use %s, %d for example, just place the vars after the string with a comma.

EDIT: playerid isn't a string either, if you wanted names it'd be;

pawn Код:

new pName[MAX_PLAYER_NAME], string[128];
pName = GetPlayerName(playerid, pName, MAX_PLAYER_NAME);
format(string, sizeof(string), "Congratulations you killed %s !", pName);
SendClientMessage(killerid, COLOR_YELLOW, string);

EDIT again: Thanks for the post below, forgot about format.