the same problem here. I made a brandnew install of WIN10 64bit (Version2004), I installed P3D V.5.1 and it works without any problem.
When I run the FSUIPC Installer (6.0.12) it tells me: ""Cannot find P3Dv4 or P3Dv5 installation and so cannot install".
- I reinstalled the client of P3D: No change
- I scanned the registry with regedit and it seems, there is no information about the location of P3D (programm files, standard location)
It's mysterious to me... Any idea what I can do?