[Sawalha]

You have set the function, that's okay, but you forgot the callback which you are gonna use to call this callback:
You can use a timer , or you can use OnPlayerUpdate callback:

pawn Код:

public OnPlayerUpdate(playerid)
{ 
        new Float:health;
    GetPlayerHealth(playerid, health);
    if(health < 6)
    {
        TogglePlayerControllable(playerid, 0);
        SendClientMessage(playerid, COLOR_WHITE, "[Medic]: Nu mai ai Viata, Contacteaza un Medic(/call 911).");
    }
    return 1;

OnPlayerUpdate is called every second, when player's stats are updated.

EDIT: nvm, late.