This was a loong time ago, but lets say you have installed P3D on i.e D:\Games\P3D in 2014 with PMDG 737. Then later for some reason you uninstall P3D and PMDG in 2016 (not reinstalling Windows) and reinstall P3D again, but this time to D:\P3D, then you might get this issue when installing PMDG because it is still looking for P3D in D:\Games\P3D.
Hope this helps