In the current message one can read:
Your bounty on question "..." is completed. You must award it to an answer within 24 hours.
To be clear this is the message that appears if I didn't give my bounty to an answer within the bounty period (7 days, if I am not mistaken).
I think this message is faulty because the bounty is not completed (I did not give the bounty to anyone, and I still can award it to an answer).
What I am trying to convey is that the first sentence is somewhat contradictory to the second sentence.
Here's my suggestion:
You should award the bounty of the question "..." to an answer within 24 hours. This is the last reminder.
Additionally a link to how bounties are automatically awarded if the user chooses to not award it could prove useful.