There are certain requirements to be able to set a bounty, as [explained in the FAQ][1].

> There are a few other rules around bounties:
> 
> - **Questions must be at least 2 days old to be eligible for a bounty.**
> - Users must have at least 75 reputation to offer a bounty.
> - Only 1 active bounty per question at any given time.
> - There are a maximum of 3 active bounties allowed per user at any
> given time.
> - Once initiated, the bounty period lasts 7 days.
> - After starting a bounty, you must wait 1 day before awarding it.
> - There is a grace period of 24 hours after a bounty ends to manually
> award the bounty.
> - If you do not award your bounty within 7 days (plus the grace
> period), the highest voted answer created after the bounty started
> with at least 2 upvotes will be awarded half the bounty amount.


  [1]: http://superuser.com/faq#bounty