Ahh yeah that's a bit confusing, the merge topic tool can be used between categories, so you should click merge first and then select the topics. The placement seems like a theme/skin issue.
Merge categories based on tags
I have kinda unusual case: multiple topics can belong to one or more categories, so I decided to use combination of tags and categories - because by default in NodeBB topic can belong only to one category (which can have at most one parent).
Example: Let's imagine a topic which belongs to following categories: technical support, nodebb plugin development and nodebb development. Main or primary category would be technical support, and tags would be technical-support, nodebb-plugin-development, nodebb-development.
How to display that topic also in categories nodebb plugin development and nodebb development?
I found following plugin and was wondering if it was possible to display topics category with tag with same name:
PitaJ Global Moderator Plugin & Theme Devreplied to metalkramp on last edited by PitaJ
@metalkramp it sounds like it may be easier for you to only use tags in this case, and use a custom plugin to display certain tags as if they are categories.