I can’t add more categories

In order to be able to help you, we would need some more information from you. Which WordPress version are you using? Which editor are you working with? You are also welcome to show a screenshot, see: https://www.ads-software.com/support/forum-user-guide/block-editor/#adding-screenshots

I using Version 6.7 and Classic Editor

I have a post with category A, B, i’m editing the post and add category C, then updated, but nothing changes, the post still keeps category A, B but C is not added

Thanks for the details. I have tried to recreate it, unfortunately without success. Adding new categories from the post works fine for me.

Did you also click on the button to create the category and not just on the button to update the post?

I would also recommend deactivating all plugins to rule them out as the cause (except for the Classic Editor plugin).

Here the same technical problem with the classic editor after updating to version 6.7. Most posts assume the default category instead of the category selected by the user writer.

I deactivated all plugins but still problem, the post only default category, can’t add more category

P/s: When I select category in All Categories section, it works, but when I select category in Most Used tab, it doesn’t work.