Confirm that end stop switch is loosely held and can move sideways.

Slide slider block on linear rail up against the top end stop switch holder plastic

With your fingers, wriggle the switch around until you hear the switch activate with an audible "CLICK"

While holding the switch in place, tighten the two M2 x 14mm SHCS holding the switch

Verify functionality by sliding the slider block down and back up again. You should hear the switch click.