When I looked a few seconds ago, I couldn't find a start a bounty link for this question.
Is there a reason for this?
You have awarded the following bounties already on the post in question:
Bounties, when chosen, have to increase at least two-fold:
We now double the minimum "spend" for successive bounties on the same question by the same user. So if you started at 50: your next bounty on that question has a minimum of 100, the next 200, the next 400, the next (max).
You started out with awarding the maximum 500, so successive bounties can only be awarded at 500 reputation (the maximum). This is obviously doable by nature of your almost 10K reputation splurge. However, you can also only award the bounty when you have enough reputation. Currently you're at 140:
Try again when you reach 500. Note that with reputation loss comes privilege loss as well.
Reference: How does the bounty system work?