Troubleshooting .NET Framework 3.5 Installation Error 0x800f0818 on Windows Server 2008 R2

Follow these steps to address the installation error 0x800f0818 for .NET Framework 3.5 on Windows Server 2008 R2:

1. Ensure the .NET Framework 3.5 feature is activated on Windows Server 2008 R2. Navigate to Server Manager > Features > .NET Framework 3.5. If deactivated, activate it and attempt the installation once more.
2. Verify the availability of the installation source, which could be either a DVD or an ISO file. Confirm that the source is accessible.
3. Examine any pending updates or required restarts on the server. Complete all necessary updates and reboot the server as needed.
4. Utilise the DISM command for the installation of .NET Framework 3.5. Launch Command Prompt with administrative privileges and execute:

 dism /online /enable-feature /featurename:NetFx3 /all /source::\sources\sxs /LimitAccess

Substitute with your source drive letter.

5. If the previous methods fail, consider repairing the Windows Server 2008 R2 installation via the installation media. Initiate the system from the media and choose the repair option.