Most likely cause, your windows registry keys for P3D has been corrupted or is not valid anymore, maybe you used the P3D migration tool ?
Also, are you trying to install into the root of the drive ? That's a huge security hole, is really not a good idea, and many products will fail if the sim is installed into the root folder of a drive, such as C:\ or D:\
If you did that, uninstall the sim, and reinstall it into a normal folder, not on the root.