That is weird, especially because there is only one answer to that question.
If I read the Help Center: https://wordpress.stackexchange.com/help/bounty The bounty should have been awarded to this one question.
The thing is, did the answer had a minimum of 2 votes at the duration of the bounty? (This is crucial)
If it's indeed the case, you could try to contact a moderator by flagging the post and ask them to check if there's something going wrong.
How is a bounty awarded?
The bounty period lasts 7 days. Bounties must
have a minimum duration of at least 1 day. After the bounty ends,
there is a grace period of 24 hours to manually award the bounty.
Simply click the bounty award icon next to each answer to permanently
award your bounty to the answerer. (You cannot award a bounty to your
If you do not award your bounty within 7 days (plus the grace period),
the highest voted answer created after the bounty started with a
minimum score of 2 will be awarded half the bounty amount (or the full
amount, if the answer is also accepted). If two or more eligible
answers have the same score (their scores are tied), the oldest answer
is chosen. If there's no answer meeting those criteria, no bounty is
awarded to anyone.
If the bounty was started by the question owner, and the question
owner accepts an answer posted during the bounty period, and the
bounty expires without an explicit award then we assume the bounty
owner liked the answer they accepted and award it the full bounty
amount at the time of bounty expiration.
As a special bonus, any reputation you earn from being awarded a
bounty is exempt from the daily reputation cap.