Bug where you couldn’t add permissions to sub-category even though parent category is accessible by everyone was fixed few weeks ago (https://github.com/discourse/discourse/commit/fea5d4f3eceba37c43559a7c61a962de7d41d3b3)
You have to make sure group 1, group 2 etc. have permissions to see parent category before adding them to sub-categories.
I improved error message so it’s clearer now which groups only have access to sub-category and should be added to parent category:
https://github.com/discourse/discourse/commit/88128f1cedd069cbdf751bdee7fc8727355b06fb