[Schneider]

You can't format a string within the ShowPlayerDialog-function. You first have to format the string and then show the string in that function:

pawn Code:

CMD:credits(playerid, params[])
{
    new pname[MAX_PLAYER_NAME], str[280];
    GetPlayerName(playerid, pname,sizeof(pname));
    format(str, sizeof(str),"Scripters: StuunG23\nMappers: StuunG23\nTesters: PedroX_, Syntec,Yannick, Ultrascripter\nSpecial Thanks to: Pedrox,Syntec,Ultrascripter, Yannick,Dopeboy,Dynamite\nAnd for the sa-mp development team. and thank you %s for playing in our server", pname);
    ShowPlayerDialog(playerid, 1, DIALOG_STYLE_MSGBOX, "Credits", str, "Ok","");
    return 1;
}