I have at least 75 rep. I don't see any link underneath the comments to start a bounty:
You have posted an answer to that question. That makes the minimum bounty you can set 100 reputation, not 50. This is to avoid people posting bounties to draw attention to their answers, rather than the question. From the help center:
- To avoid overly promotional bounties, if you are offering a bounty on a question that you have already posted an answer to, your minimum spend is 100 reputation (not 50).