This is not a bug, it is by design. You have to double the amount of your previous bounty for subsequent bounties on the same question. This mechanism is in place to avoid placing multiple cheap bounties on a question in order to maintain high visibility for it.
So, since you already awarded a bounty of 50 rep on that question your next bounty on that question must be at least 100 rep, which you can't give yet. Get yourself over 100 rep and you should be able to add another bounty.