How to solve the error code 0x800f0818: Error installing .NET Framework 3.5 in Windows Server 2008 R2

There are several steps you can take to solve the error code 0x800f0818 when installing .NET Framework 3.5 in Windows Server 2008 R2:

1. Check if the .NET Framework 3.5 feature is enabled in Windows Server 2008 R2. To do this, go to Server Manager > Features > .NET Framework 3.5. If it is not enabled, enable it and try installing again.
2. Check if the installation source is available. The installation source can be a DVD or an ISO file. Make sure it is available and accessible.
3. Check if there are any pending updates or pending restarts on the server. Install any pending updates and restart the server if necessary.
4. Try installing .NET Framework 3.5 using the DISM command. Open Command Prompt as an administrator and run the following command:

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

   Replace <drive> with the drive letter of the installation source.

5. If none of the above steps work, try repairing the Windows Server 2008 R2 installation using the installation media. Boot from the installation media and select the option to repair the installation.