Ok to make something up on behavioral interview questions (tell me about a time when...)?

I’ve been doing interviews for a while now (26) and have practiced endlessly. I can easily give my opinion on things or answer any type of question just fine. However, for some reason my mind always goes blank when asked to discuss a specific example for behavioral questions. Every. Single. Time. I just can’t think of anything from past work experience on the spot. Is it ok to make up a situation?

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