[dennissk]

It's something to see, the String is above the Callback, you should leave it.

public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid)
{
new string[256];
format(string,sizeof(string),"-%.0f hp",amount);
SetPlayerChatBubble(playerid,string,0xFFFFFF,150.0 ,2500);
return 1;
}