I see no link for this.
There is normally a "start a bounty" link underneath the question.
However, in this particular case, you previously offered a bounty of 100 on this question:
According to the bounty rules:
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 so on).
So the minimum bounty you can now offer is 200 (double the previous 100) and you currently have 81 rep.
This is presumably why you can't see the "start a bounty" link.