Why when setting a bounty here in this SE the minimum is 100 rep whereas on other SE sites they have a 50 reputation option?
What is the minimum amount of rep I can offer a bounty for?
For most questions, 50 reputation. There are some exceptions, though:
- If you have already answered the question before, the minimum bounty offer is 100.
- If you have already offered a bounty on the question before, the minimum offer is double your last offer (see below).
I would guess from your recent activity that you're attempting to place a bounty on this question, which you have posted an answer to.
These adjustments are in place to prevent abuse of the bounty system by making it expensive to perpetually promote a question or to draw attention to your own answers.