Can't see a link to add a bounty either. Doesn't matter that it already has an accepted answer, they're no longer tied to the bounty.
Currently, the bounty exclusion rules are:
You already have an active bounty on any question (only one is allowed)
No, never set a bounty in the system. That exclusion doesn't apply.
Lacking sufficient rep to start a bounty (100 is the minimum)
At 3000+ that's not the problem in this case, some 2900+ over the 100 minimum required to set even the smallest of bounties.
Question is not yet 2 days old (the minimum age)
That question was asked six months ago, so that's not the problem either.
But then maybe that is the problem.
The change in the bounty system was announced two months ago and may be an edge condition where the new and the old settings of bounties just aren't quite getting along.