The correct way to draw a string with
var font = load('res://Font.tres')
var pos = Vector2(500, 500)
var string = 'Test'
for i in string.length():
var c = string[i]
var next_char = string[i + 1] if i + 1 < string.length() else ''
var advance = draw_char(font, pos, c, next_char, Color.red)
pos.x += advance
draw_char only draws one-character string, which is why you don't see anything.
I don't recommend using this for drawing a string, but instead use draw_string:
draw_string(font, pos, 'Test', Color.red)