[newbienoob]

pawn Код:

public OnPlayerDeath(playerid, killerid, reason)
{
    new killername[24],playername[24];
    GetPlayerName(playerid,playername,24);//get player's name
    GetPlayerName(killerid,killername,24);//get killer's name
    SendDeathMessage(killerid, playerid, reason);
    SetPlayerScore(killerid,GetPlayerScore(killerid)+1);
    new weapname[50]; //brought it before the getplayer
    new string[128]; //defined a string
    GetWeaponName(GetPlayerWeapon(killerid),weapname, sizeof(weapname));
    format(string,128,"%s(%d) has killed %s(%d) with a %s.",killername,killerid,playername,playerid,weapname);
    SendClientMessageToAll(0x800000AA, string);
    return 1;
}

You forgot to add GetPlayerName..