Why do I get the message You are not allowed to create or update this content

Because you are not allowed to create or update that content. That's my guess.

Well, maybe we also need a "smart" answer. You've provided zero information from which you expect us to provide an answer? Maybe had too much weed today, Bob? 😁

What are you trying to update and where are you trying to update it?

I had the same problem. It was solved when I selected the community I wanted to post to (selected as opposed to just writing it myself), as well as some sub-section shown with some boxes to chose from.

you are being a complete jerk to this man who is asking a normal question - I came here to get the same answer to this man's question.

well that helps - at least now we know you're talking about posting on these forums. The original post has no such details and it wasn't clear where the error message was coming from.