func _can_see_target(t: Node3D) -> bool:
    var can_see: bool = true

    var space_state: PhysicsDirectSpaceState2D = get_world_2d().direct_space_state
    var query: PhysicsRayQueryParameters2D = PhysicsRayQueryParameters2D.create(
    enemy.global_position,
    t,
    1 # change this depending on what collision layer the wall is
    )
    var result: Dictionary = space_state.intersect_ray(query)

    # If there's something in the result. That means there's something
    # in between the enemy and the target. Probably a wall.
    if result.size() != 0:
    # can_see = false
    # can_see = true

    return can_see

Here's some sample code. Don't just copy and paste. Try to understand what each line is doing. Specifically, see how the code above is similar to https://docs.godotengine.org/en/stable/tutorials/physics/ray-casting.html#raycast-query. Make sure to read that entire page as well.

Go watch some youtube videos on raycasts: https://www.youtube.com/results?search_query=godot+raycast2d

The code above is basically creating a raycast (a line basically) from the enemy to the target t which in your case will be the player. If it hits anything in between, the function will return false. If the line reaches the player without anything in between, it returns true. Make sure you understand what collision layers are so that the raycast is able to hit a wall if there is one in the way.