[Scenario]

You had it set to show the dialog to the player which the admin clicked on. You also had something above the "IsPlayerConnected" check which didn't make any sense to be there - so I removed it and changed the "IsPlayerConnected" to what it should be.

pawn Код:

public OnPlayerClickPlayer(playerid, clickedplayerid, source)
{
    if(IsPlayerAdmin(playerid))
    {
        ShowPlayerDialog(playerid, DIALOG_STYLE_LIST, "blablabla", "Force change class", "Ok", "Cancel");

        if(!IsPlayerConnected(clickedplayerid)) return SendClientMessage(playerid,0xFF0000FF,"Invalid ID");

        new message[128];
        format(message, 128, "You clicked on player %d", clickedplayerid);
        SendClientMessage(playerid, 0xFFFFFFFF, message);
        return 1;
    }
    return 0;
}