If I go to a category in my forums, all the topics are the name of that category.
Sounds like you have one of my old plugins installed, give me a few minutes and I will push an update.
Update nodebb-plugin-rainbows, it should work now.
@hnahli there is a daily/weekly, etc digest email option available
From a category view (list of topics) you can go to tools -> merge to merge new topics into old ones.
Its possible to assign a group the moderating power on just one category, However be aware that this will also include other moderation powers such as Forking, Closing and Moving topics.
I think that function was removed, you might have some old code that is still calling it. What is your NodeBB version and the rest of the stack trace?