Error 421 - Invalid directory

[tsGucci]

Invalid directory - Can not install the scenery. / Error 421. Can not find the FSX/Prepar3D root folder in the registry!

The installer can not find the value which stores the FS2004/FSX/Prepar3D location. Most probably the registry entry which contains the simulator's root folder is missing or broken. You can find  a program at Flight1 webpage they have a tool called "FSX/FS9 Registry Repair Tool (36KB)" http://www.flight1.com/view.asp?page=library  Download and run it, select the appropriate folder. It should fix the registry. It is important to run it as administrator (right click on the file and choose: Run as administrator)
Re-install the scenery.