I had the same issue as well, I tried deleting all the files and rebuilding but it still didn’t work.
Later I noticed in VS output log “XYZ.generated.h file should always be the last”, I noticed that I had added another #include line after #include “XYZ.generated.h” which caused the error.
Since this thread is the first result when I google this problem, I figured I will also provide my solution to this problem.
For me, the problem was that I accidentally included the class prefix in the file name (i.e. I had a class called “FMyStruct”, and I accidentally called the file name “FMyStruct.h”). This breaks the build system, eventually I noticed this problem and corrected it by changing the file name to “MyStruct.h” (note that it no longer includes the ‘F’ prefix).
Thank you! It will be useful for newcomers. I am one of them 
This was also my problem. The problematic file was listed one line above the error in Rider’s build log. It seems for some reason I had an include line in my Character.cpp file for a random header from the engine source that I wasn’t even using. Not sure how it got there. In any case error went away after removing it
I think this is my issue, I was going to backup my files and accidentally moved my content folder out of the project. Since then the project wont even open, Ive deleted all the folders besides config, content, and source and rebuilt the project and solution but nothing has worked. Any ideas?