How to make question into Bounty question if that user dont have enough credits ..
If you don't have enough reputation... you can't make it a bounty question. That's rather the point of the restriction, isn't it? Like anywhere else that you spend some quantity for a service, if you have insufficient funds you can't get the service, plain and simple.
If you mean how to set up a bounty on someone else's post because that someone else doesn't have enough reputation, you cannot currently, but it's a very popular feature request.