[Kwarde]

It kinda means your code looks messy.

pawn Код:

//Bad indentation:
CMD:mycommand(playerid, params[])
{
    if (IsPlayerAdmin(playerid)){
        SendClientMessage(playerid, -1, "Hello RCON admin!");
        return 1;
      } //This one; it should be at the same vertical line as the if statement
      SendClientMessage(playerid, -1, "Hello player!"); //Samewise..
    return 1;
}

//Good indentation:
CMD:mycommand(playerid, params[])
{
    if (IsPlayerAdmin(playerid))
    {
        SendClientMessage(playerid, -1, "Hello RCON admin!");
        return 1;
    } 
    SendClientMessage(playerid, -1, "Hello player!"); 
    return 1;
}

By the way,

1- it is really not needed to check if the player is online or not.. if a player is offline they cannot use the command thus it will not be called
2- your command stops (returning 1) if the player using the command is online, so you'll never be able to use that command

EDIT:
@David{somethingelse};
Please consider explaining WHY he had the warning. If you don't explain what was wrong, they'll never learn.