As you can see in the pic I have 7 fuel, it says it cost 7 fuel, but says not enough fuel.
it’s a bug, if you refresh you’ll see you actually have less than 7 fuel
OR the other bug where you can’t have less than 1 meaning it won’t show zero so it claims you don’t have enough fuel.
If you need 7 you require 8 in your tank etc etc.
Never had any problem going down to 0 fuel.