Should I be able to set a bounty on a question asked 2 days ago if I have 65 rep?
Sadly you need a reputation of 75 or higher to set a bounty.
It seems you've bountied another post which lowered your rep below the 75 point threshold.
It may seem counter-intuitive but permissions are revoked if you don't meet the point threshold necessary. You must maintain your reputation to continue to enjoy certain site features.
See here for other permissions earned at various levels: https://magento.stackexchange.com/help/privileges/