Learn Italian: Ciascuno ha un difetto, ma noi lavoriamo insieme. - Each one has a flaw, but we work together.

Ciascuno means "each one" or "each person." Even though it refers to a whole group, it singles out the individuals. Just like in English where we say "each one has" (not "have"), Italian pairs ciascuno with a third-person singular verb like ha.

Could I use "ognuno" instead of "ciascuno" here?

Yes! As indefinite pronouns, ognuno (everyone) and ciascuno (each one) are basically interchangeable. Ciascuno ha un difetto and ognuno ha un difetto mean the same thing, though ciascuno puts a slightly stronger focus on the individuals rather than the group as a whole.

Why not just say "Tutti hanno un difetto"?

You can! Tutti hanno un difetto (Everyone has a flaw) is perfectly natural. However, using ciascuno (each one) sets up a nice stylistic contrast in this specific sentence: despite our individual flaws, we work together (insieme) as a team.

Why is the pronoun "noi" used before "lavoriamo"? Isn't it usually dropped?

Subject pronouns are usually dropped in Italian, but here noi is added to create a strong contrast. The sentence is comparing the individuals who have flaws (ciascuno) with the group as a whole. Adding noi emphasizes that "WE" overcome those individual flaws to work together.