Your cash register probably looks like in image 1, the relevant button in light gray. Image 2 shows what the button should look like when it works. For this button 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: 'Custom price' must be checked in the POS settings.

If all went well, the button should now work.