I can't find an option to answer any question which is protected. As it says that at least 10 rep is required but I already have 51 rep. Why is it so? Example:
Question: How can I keep my cat off my keyboard?
The reputation required to answer a protected question ignores the +100 account association bonus that comes with having an account on another site with enough reputation. Instead, you must earn 10 reputation on this site through questions and answers. See here for more info on protected questions.
Anyone who joins a site with association bonus rep needs to have 110 rep to answer protected question.
Since you put a 50-rep bounty, reducing your rep to 51, you need to earn 59 rep (not 10) to pass the requirement to answer protected question.
This might be a bug, as reported on MSE: Stack Exchange doesn't allow me to answer protected questions because I gave away earned rep in bounties