If you can't answer because you're drawing a blank just say you'll think about it and ask to go onto the next question and come back to this one later if you think of something.
No one should expect that all behavioral interview questions will be answerable by everyone. People simply don't index their memories that way. You can "punt" on perhaps one or two questions, but if it you're doing that on more, then you'll need to spend more time preparing yourself before the next behavioral interview.
You can prepare in advance to some extent by simply reviewing lists of behavioral interview questions and coming up with some good answers without the pressure of a live interview. There's not an infinity of such questions, there's probably a few hundred different questions each with some easy variations. The answer for each of these requires illustrating a particular trait with some real example drawn from previous experience.
While coming up with answers, it might be useful to consider the overall goal of the behavioral question. These types of interviews are intended to discern what it is like to work with you as a person. When things are going perfectly in a workplace, almost everyone is remarkably easy to get along with. But when there is conflict or pressure, that's when personalities clash and when getting along with others becomes paramount above even things like technical aptitude. That is why behavioral interview focus almost exclusively on how you deal with stressors, mistakes and problems involving other people.
What is the goal of this particular question? I believe it is intended to probe empathy and the ability to criticize oneself (to admit to being wrong).
The question asks for an explanation about a time when you should have apologized but did not. You could come up with something silly like a grade school incident where you pulled a girl's pigtails where the explanation would be that you were 6 years old or perhaps some situation where you did something trivial that would typically require an uttering "excuse me" but did not. That's NOT what the question is intended to probe.
A really good answer to this question would describe some event where you truly failed to apologize for something that seriously required an apology. If you then indicate some amount of regret and explain it by describing the point of view of the person you wronged that would show that you have empathy and can admit to being wrong.
There's always a lot of "wiggle room" in behavioral questions-- maybe you can't think of a time where you failed to apologize, but you can probably think of a time where your apology was delayed. That would work. Or you can perhaps explain how you made amends to someone for a mistake without apologizing. Sometimes actions speak louder than words ( :-) ).
Behavioral interviews are exceptionally difficult and NOT just for the interviewee.
Part of the problem here lies with the interviewer. It takes a lot of skill to perform a behavioral interview properly. A really good behavioral interviewer will run the interview much like a conversation and you may not even notice that it is a behavioral interview. A poor one, will simply pick a trait and ask a question starting with "Tell me about a time where..."
Skilled interviewers start with a discussion about previous jobs/projects/experiences and then proceed to ask behavioral questions within the context of the discussion. The interviewee is far more likely to be able to answer behavioral questions when it is asked within the context of a specific discussion about a past event.