I want to make a bounty on a question and I have seen some questions currently with a 50 rep bounty, but the only option available to me is a 100 rep bounty. I was wondering why.
If you've already given a +50 bounty on a question, the way the system works is that you're allowed to put up more bounties on the same question, but they need to be double the amount of rep of the previous bounty.
So if you put up a first bounty of +50, then the next bounty must be +100. Then the next one must be +200, and so on, up to a maximum of +500.
See how does the bounty system work? on MSE for all the details.