Kmy appimage (5.1.0-5ce7527)
Ubuntu 20.04
Appearance: 5 categories viewed as empty
Action: right click on each category gives normal drop down menu but delete option is greyed out.
I searched on Google and read various posts but suggested solutions didn’t seem to work for me. This included checking scheduled transactions and reports for any associations. When none were found I decided that this was something I had to live with.
However, I stumbled on the following workaround:
As the 5 stubborn categories were scattered in the category list, I created a new parent category which I simply called ‘empty’ and placed the 5 stubborn categories inside. My motive was merely to tidy things up.
I then wondered what would happen if I then tried to delete my newly created parent category. To my surprise, the delete option for this category and the 5 stubborn sub-categories was now accessible. I’m no expert, so unable to explain my stumble. Look forward to reading an explanation from those who are more technically able? My apologies if this issue has been fully addressed previously.

Just to clarify, you had several categories which you couldn't delete. You confirmed they were not referenced in any scheduled transactions or reports. You created a new category, and moved the problem ones under the new one. Once you deleted the new one, the ones which were below it were now at the top level, but you were able to delete each of them?

No, it doesn't make any sense to me how that could happen, but knowing the details may help track down the actual cause.

All above clarification was correct apart from this “ ..… the ones which were below it were now at the top level …..”. What I did was to right-click on the new category which brought up the category options dialogue. I then clicked on delete, this opened a new dialogue which asked me if I also wanted to delete the sub-categories. I clicked on yes and all was gone. Sorry about the lack of technical details.

That's actually good information. It sounds to me like there might be a bug in there somewhere. Deleting a category should not delete a sub-category if it doesn't meet the criteria for deletion, or else it should have been possible to delete it by itself. Do you still have a backup of the file that shows the problem? If you do, one quick test would be to see if you can delete one of the problem categories by itself, after moving it under the new category. (If you do this, rename the file to something new, so you don't have a chance of confusing it with your good working file.)

Just tried this but unable to delete problem category by itself, delete option remains greyed out. The option of deleting the problem category only possible via deletion of the main category.

Can you create a anonymous version of your data file for further analysis? The manual contains instructions. Send the anonymized file to the KMyMoney developer mailing list at kmymoney-devel@kde.org with some instructions on which category causes the problem.

Okay, if I've followed the instructions correctly, you should now have the file.