Your cash register probably looks like in image 1, the relevant buttons in light gray. Image 2 shows what the buttons should look like when they do work. For these buttons to work correctly, a number of settings must be correct.

1. Admin → Tickets → Price types → Edit: 'Free amount allowed' must be checked.

2. Admin → Settings → Admin access → Roles → Edit: ‘ Discount’ and 'Custom price' must be checked in the POS settings.

If all went well, the buttons should now work.