D:\Server\gamemodes\classic.pwn(165) : error 017: undefined symbol "ReturnUser"

    new cmd[256], idx;
	cmd = strtok(cmdtext, idx);
	if(strcmp(cmd, "/faadmin", true) == 0)
	{
       new string[128];
       new tmp[256];
       new player[MAX_PLAYER_NAME], giveplayer[MAX_PLAYER_NAME];
       new giveplayerid;
       if (IsPlayerAdmin(playerid))
       {
               tmp = strtok(cmdtext, idx);
               if(!strlen(tmp))
               {
                       SendClientMessage(playerid, 0xFF0000FF, "Foloseste: /faadmin [playerid] [level]");
                       SendClientMessage(playerid, 0xFF0000FF, "Functie: Jucatorul va fi admin.");
                       return 1;
               }
               giveplayerid = ReturnUser(tmp); - Here the problem !
               tmp = strtok(cmdtext, idx);
               new level = strval(tmp);
               if(giveplayerid != INVALID_PLAYER_ID)
               {
                       GetPlayerName(giveplayerid, giveplayer, sizeof(giveplayer));
                       GetPlayerName(playerid, player, sizeof(player));
                       PlayerInfo[giveplayerid][pAdminLevel] = level;
                       printf("Admin %s l-a facut pe %s nivel %d de admin.", player, giveplayer, level);
                       format(string, sizeof(string), "Ai fost facut nivel %d de Administrator de catre %s.", level,  player);
                       SendClientMessage(giveplayerid, 0x00C2ECFF, string);
                       format(string, sizeof(string), "L-ai facut pe %s nivel %d de admin.",  giveplayer,PlayerInfo[giveplayerid][pAdminLevel]);
                       SendClientMessage(playerid, 0x00C2ECFF, string);
               }
               else if(giveplayerid == INVALID_PLAYER_ID)
               {
                       format(string, sizeof(string), "%i nu este activ.", giveplayerid);
                       SendClientMessage(playerid, 0xE60000FF, string);
               }
       }
       else
       {
           SendClientMessage(playerid, 0xE60000FF, "Nu esti Proprietar!");
       }
       return 1;
	}