The help centre says the following about bounties:
Additionally, if you offer multiple bounties on the same question, the
minimum spend doubles with each subsequent bounty (50 reputation on
the first bounty, 100 reputation on the second, 200 on the third, and
This is slightly misleading as it gives the impression that the first bounty is ...
Did you wait 24 hours after starting the bounty before trying to award it? There's a waiting period before you can give it out.
From the FAQ on Meta Stack Exchange:
How do I award a bounty?
A bounty can be awarded 24 hours after the bounty was started.
You know, this idea actually makes sense to me. Eight years ago Jeff Atwood said:
This is effectively completed, because
bounty system is no longer tied to accepting an answer in any way
you can issue multiple bounties on your question if necessary
Let's think about this. Stringing together a series of four +50 bounties gives you an effective bounty ...
From the bounty page in the help centre:
If you do not award the bounty within 24 hours of the bounty period
ending, half the bounty value will be automatically awarded to the top
voted answer posted after the bounty start, provided it has a score of
at least 2. If no new answer matches this requirement, no reputation
will be awarded at all, and ...