Bug lesson 18-practice about learn-gdscript HOT 1 OPEN

This is not a bug. You have 2 weird things in your code.
This is the code that is responsible for selecting the units in the background of the live editor :

func select_units(cells: Array) -> void:
	selected_units.clear()
	for cell in cells:
		if cell in units:
			selected_units.append(cell)
	update()

You did not need to know this to actually complete the exercise but its better to show you.
What your code does is to call the func 4 times with 4 different arrays instead of doing it only once. Your code would have worked if the code above would not have had selected_units.clear() but this is beyond the purpose of this explanation.

select_units([1,0])
select_units([5,1])
select_units([4,2])
select_units([0,3])

Instead of this try calling it in a proper manner something like : select_units([Vector2(x, y), Vector2(x, y), Vector2(x, y), Vector2(x, y)]).

Second thing you understood wrong is that you're trying to call select units with an array containing simple numbers as each index instead of a Vector2 which holds coordinates. Please revisit some of the chapters prior to Lesson 18 for a better understanding.

from learn-gdscript.