I just ran into this right now. It was a pain because the window that pops up doesn’t stay open if there’s an error in the log, but to see what’s going on you can run the following command line:
This is the command line that gets run when you right click your uproject and generate project files. If you run this from a command line window you’ll be able to see the program’s output and try to fix the errors you’re getting. For me, some of the engine plugins I depended on for my 4.15 project had been renamed and this was preventing the project from being generated.