[Dutheil]

So, the error "undefined symbol" means you use a function or variable that you haven't declared.
Here, you forgot to declare the variable "sendername". You need to declare it before use it.

I advise you not declare it in your loop as did my upstairs neighbor.

PHP код:

if(strcmp(string, "/admins", true) == 0)
{
    if(!IsPlayerConnected(playerid)) return 1;
    
    SendClientMessage(playerid, COLOR_LBLUE, "Admins Online:");
    
    new
        sendername[MAX_PLAYER_NAME+1];
    
    for(new i = 0; i < MAX_PLAYERS; i++)
    {
        if(!IsPlayerConnected(i)) continue;
        if(adlvl[playerid] > 0) continue;
        
        GetPlayerName(i, sendername, sizeof(sendername));
        format(string, 256, "[Level:%d] %s (ID:%d)", adlvl, sendername, playerid);
        
        SendClientMessage(playerid, COLOR_GREEN, string);
    }
    
    return 1;
}