Thanks a-million! This is what got me sorted out. I believe I had done a full compile of the UE4 solution, but maybe it didn’t include the projects folder. The first time I have ever tried to build lighting with a source version of the editor. This is a reliable failure of lightmass with UESource. I had the same issue back when I had 4.22S, but didn’t dig deep into the fix until now.
For those having other issues with lightmass, there is good documentation here. It even goes over cleaning and re-validating cache, as well and having to compile the UnrealLightmass solution from within Visual Studio, for UES.