Why I am getting "You can accept the answer in 2 minutes" while I try to accept the answer on Ask Ubuntu.
Adding an answer so this question doesn't go unanswered..
The Stack Exchange system prevents accepting an answer within the first 15 minutes of asking the question. This is the system's way of ensuring that good to great answers are made.
If an answer is made to your question within the first 15 minutes of asking, but is not particularly good but you still accept it, it discourages others from adding an even better answer. In order to prevent that, this check is in place.
Relevant Meta Stackoverflow question: