last_reward
Status 409. The store's only reward cannot be removed.
What it means
This is the store's only reward. A programme with nothing to spend points on is a trap: points pile up and buy nothing. Nothing was changed.
What to do
Add another reward with POST /stores/{store_key}/programme/rewards first, then remove this one with a new Idempotency-Key.
Safe to send again?
Not as it is. Change what the answer points to, then send the request.
Where you can meet it
POST /stores/{store_key}/programme/rewards/{reward_id}/remove: Remove a reward
Every code, and the shape of an error answer: Errors.