How to solve the error code 0x800f0831: Failed to install .NET Framework 3.5 in Windows Server 2019

There are several steps you can take to solve the error code 0x800f0831 when trying to install .NET Framework 3.5 in Windows Server 2019:

1. Check if .NET Framework 3.5 is already installed: Open the Control Panel and go to Programs and Features. Look for Microsoft .NET Framework 3.5 and check if it is already installed. If it is, you don't need to install it again.
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 2019 installation media. This command will install .NET Framework 3.5 from the installation media.
3. Use the Windows Update Troubleshooter: Open the Settings app and go to Update & Security. Click on Troubleshoot and select Windows Update. Run the troubleshooter and follow the instructions to fix any issues with Windows Update.
4. Check for Windows updates: Open the Settings app and go to Update & Security. Click on Check for updates and install any available updates. Sometimes, installing the latest updates can fix issues with installing .NET Framework 3.5.
5. Use the .NET Framework Repair Tool: Download and run the .NET Framework Repair Tool from Microsoft's website. This tool can fix common issues with .NET Framework installations.