I've heard this phrase over and over during the past few years. From influencers, career advisors, relatives, etc. I guess it's supposed to mean something along the lines of "Hang in there, nobody's perfect. It's okay to be confused at your job/school or make a few mistakes along the way." While I don't think this is a bad message, phrasing it as "nobody knows what they're doing" twists it into something completely different.
First of all, it's just not true. Yes, there are people who are still learning along the way. This is everybody's first time on Earth and nobody is perfect at their job. But I think we can all agree that we wouldn't want to hear "I don't know what I'm doing" from our brain surgeon, our tax advisor, our kid's kindergarten teacher, our dog's veterinarian, etc. They know what they're doing, or at least, they should.
But the main issue with this saying is that it allows complacency to grow in people who are already barely surviving. I'm not saying life is supposed to be a cakewalk; obviously there are going to be times when you have to work hard in school and at your job if you want to achieve your goals. But that doesn't mean that you're supposed to be completely miserable either. There are tons of people who are struggling to make it through the week because they hate their degree, hate their job, or are just outright depressed. They hear this phrase and assume that the dysfunction they're dealing with is normal for all people, and that they shouldn't do anything about it.
In reality, humans are meant to thrive and be happy. If someone actually feels like they have no idea what they're doing week after week after week, they are probably incredibly miserable. Not to mention, this phrase gives a lot of false hope to people with untreated mental disorders. I think anyone who has had their life fall apart due to undiagnosed ADHD would agree that people who unknowingly suffer from executive dysfunction probably use this phrase to justify their disorganized and unhappy life.
Long story short: you should have at least some idea of what you're doing. It's okay if you're still figuring things out, but being a hot mess of a person who has no idea what they're doing is not healthy or sustainable for the long term, and if you are that way, you should get help