[Edit: It is my mistake, see the answer]
I posted this question Polarization formula. a bit more then 48hours ago. I reached the cap of the 75 points to give bounty. I want to start a bounty on this question. Just below to the question I see 'question eligible for bounty since 1 hour ago' but the 'start a bounty' does not appear. It is really weird since on other questions I asked (e.g. Motivation for the study of amoebas.) the 'start a bounty' button appear.
I don't understand the problem.
Edit: related question: Where is the bounty button?