I have awarded a bounty that is supposed to default tomorrow, because I really want to offer one on a more pressing question. Now that the bounty is awarded, why can't I offer it on my other question?
It might be something else.. when exactly did you ask the question you now want to offer bounty for?
It must be more than 48 hours in the past according to the bounty wiki page.
Did you read the bounty rules? Read that bulleted list VERY VERY closely: