If your Timer's name is "TestTimer", then you need to reference it with
$TestTimer. Also, even if you fix that, you'll get an error because the
text property of the Label node takes a string, and you're trying to assign a float. Finally,
time_left is a property of the Timer node, not a function.
The correct code would be:
text = str($TestTimer.time_left)
Note: This only works if "TestTimer" is a child of the node this script is on. If it's in another location, you'll need to provide the correct path to the node.