Giving your hero or heroine a fatal flaw is not always easy. However, it makes the person more human, easier to relate to, and should, of course, contribute to the plot. Everyone makes mistakes, and your characters should, too.
This fatal flaw does not have to be some horribly immoral propensity toward evil. In fact, it probably shouldn’t be, unless you’re writing a horror story. Rather, it should be something that gets him or her into trouble. It will typically be taking some trait, even one that’s usually a virtue, to an extreme. Like being too honest. Or too organized. Or too outspoken. Phobias work well, too: Afraid of snakes. Afraid of commitment. Afraid of heights.
If you need more examples, take a look at some of your favorite characters from other stories, TV shows (especially sit-coms), or movies. If you know anything about the character traits for the various astrological signs, there are clues there as well. For example, Virgo tends to be a neat-freak, Scorpio can be obsessive, Capricorn can be too ambitious, and Aries can be too aggressive, to name a few.
Such characteristics add depth, credibility, and interest which all contribute to making the people who populate your stories unforgettable.