There are several reasons why a voucher is labelled Not Available:
You are trying to redeem during an exclusion day
Vouchers are locked during exclusion days and will be labelled as "Not Available" with the note "Deals are not available on <Exclusion day>. See all exclusion days in Rules of Use."
The venue is currently closed
When the venue is closed, you are unable to redeem the voucher. If you tap into the voucher, you will see the deal as "Not Available" with the note "The restaurant is currently closed. Check operating hours on the restaurant page before visiting"
You are trying to redeem on a non-applicable day
Vouchers are locked during the non-applicable day and will be labelled as "Not Available" with the note "This voucher is not available for use today."