If I want to publish the post in a future, in a draft mode I can’t set a category at this step.
Firstly, I need to save it in the Drafts category not to publish the post immediately.
Then I edit the draft and choose the category in a special menu (not an ordinary in a composer).
When you chose the Destination category, you can edit the draft post and choose necessary tags that are loaded from the destination category. Choose smth, save it. Pay attention that while saving the chosen tags are visible and then dissapear.