My manager tried to switch off a cashier who left and the page refused. Why?

Because adding somebody, changing a PIN and switching somebody off are owner-level. Each of them hands out or takes away the right to take money, so a manager signed in to the dashboard is refused politely and told so. Sign in with the owner account, open Settings → The POS, find the person under People at the POS and press Switch off. Their name comes off the sign-in screen the next time the device checks in, and the device's own settings has a button to check again straight away. Nobody is ever deleted, so last month's sales still name them.