Nobody can sign in at a new POS

What to do when a brand-new POS will not let a cashier in.

A POS with no staff on file has nobody to let in, and says so rather than drawing a keypad that cannot work.

The fix

In the books, go to Settings → Point of sale → Users and PINs and add a user with a PIN. The POS clears itself as soon as it sees them: there is nothing to restart.