How to solve the error code 0x800f0926: Windows couldn't install .NET Framework 3.5 in Windows Server 2012

There are several steps you can take to solve the error code 0x800f0926 when trying to install .NET Framework 3.5 in Windows Server 2012:

1. Check if .NET Framework 3.5 is already installed: Go to Control Panel > Programs and Features > Turn Windows features on or off. Look for .NET Framework 3.5 and make sure it is not already installed.
2. Use the DISM command: Open Command Prompt as an administrator and type the following command: DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:D:\sources\sxs (replace D:\ with the drive letter of your Windows Server 2012 installation media). This command will install .NET Framework 3.5 from the installation media.
3. Use the Windows Update Troubleshooter: Go to Control Panel > Troubleshooting > System and Security > Fix problems with Windows Update. Follow the on-screen instructions to run the troubleshooter and fix any issues with Windows Update.
4. Download and install .NET Framework 3.5 manually: Go to the Microsoft Download Center and download the .NET Framework 3.5 installation package. Run the installation package and follow the on-screen instructions to install .NET Framework 3.5.
5. Check for system file corruption: Open Command Prompt as an administrator and type the following command: sfc /scannow. This command will scan your system files for any corruption and repair them if necessary.