Why I'm not able to create category? What's the error? What's the solution?

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I think I have fragments of memory that I had this same problem years ago.
Can't recall for sure, but I think it was related to the URL structure in Admin Panel. Try selecting one of the upper URL options and make a new attempt to create categories.

Admin > General > URL structure

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Did it work @ParYapt ? Let me know if it worked, so I can formulate an answer because this can be helpful for other people facing the same problem.