One of the prerequisites for RAD Studio is the Microsoft .NET Framework 3.5, but there is no Windows 10 .NET 3.5 installer to add this feature to the system. So I'm going to describe the few steps necessary to get .NET 3.5 into Windows 10 before installing RAD Studio.
1) Open the Control Panel and choose Programs and Features, or search for Programs and Features using Cortana
2) Choose Turn Windows features on or off
3) Enable .NET Framework 3.5 and click OK
4) Click to Download files from Windows Update and, after a (very) few minutes, you will have your Windows 10 system ready to install RAD Studio successfully
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Actually I found that this did help https://www.westechs.com/error-0x800f0954-when-installing-dot-net-on-windows-10/ but I didn't do the windows restart as this seemed to change the registry key back to 1 .. i.e. I followed steps as per https://www.westechs.com/error-0x800f0954-when-installing-dot-net-on-windows-10/ then, without doing PC restart, I then followed above steps .. this seems to have worked ..