Heather is a devout Catholic working for a taco shop in a cashier position. She formally requested an accommodation through her employer asking for $3$ specific weekends off to observe them in accordance with her closely held religious beliefs. She even found another employee who was willing to work for her on the days she was requesting, but her employer refused to make the reasonable adjustments to allow her the time requested with or without there being another employee willing to cover her on the days off she had requested. Which employment$-$related law prohibits his employer from failing to or refusing to make these reasonable adjustments?