Hey @tvavrda, thanks for letting us know. We’re working on updating this area, which should resolve the issue you are seeing here.
That work is in progress, but once it is complete, the New Topic button will be selectable even when viewing categories where you can’t create topics — the category field will be blank when that happens, and you’ll need to select a category that isn’t read-only before you can post the topic.
We’ll follow up here once that work is deployed to confirm that fixes things here as we expect it will.