[Konstantinos]

It must be inside the callback.

pawn Код:

public OnPlayerCommandPerformed( playerid, cmdtext[ ], success )
{
    new
        msg_[ 64 ],
        name_[ MAX_PLAYER_NAME ]
    ;
    GetPlayerName( playerid, name_, MAX_PLAYER_NAME );
    format( msg_, sizeof( msg_ ), "(CMD) %s: %s", name_, cmdtext );
    print( msg_ );

    for(new i = 0; i < MAX_PLAYERS; i++ )
    {
        if( !IsPlayerConnected( i ) || !IsPlayerAdmin( i ) ) continue;
        
        SendClientMessage( i, 0x754829AA, msg_ );
    }

    if( !success ) return 0;
    return 1;
}

You've already format it, just print it! No need to use printf.