Fixing Error Code 0x800f0818: Installing .NET Framework 3.5 on Windows Server 2008 R2

Addressing the error code 0x800f0818 while installing .NET Framework 3.5 on Windows Server 2008 R2 involves several critical steps:

Ensure .NET Framework 3.5 is activated: Navigate to Server Manager > Features > .NET Framework 3.5. Enable this feature if it is not already active, then attempt the installation once more.
Verify availability of the installation source: Ensure that your installation source, such as a DVD or an ISO file, is present and accessible.
Complete any outstanding system updates: Check for any pending updates or required restarts on the server. Apply updates and restart the server if necessary before proceeding.
Utilize DISM for installation: Launch Command Prompt as an administrator and execute the following command:
```
 dism /online /enable-feature /featurename:NetFx3 /all /source::\sources\sxs /LimitAccess
```
Replace with the actual drive letter where your installation source is located.
If the issue persists, consider a system repair: Use your Windows Server 2008 R2 installation media to boot and select the repair option to fix any underlying issues with the server’s operating system.