Resolving the .NET Framework 3.5 Installation Error 0x800F081F for Office 2021

To rectify the error code 0x800F081F while installing Office 2021, follow these proven strategies:

1. Method 1: Manual Installation of .NET Framework 3.5
• Hit the Windows key + R to launch the Run dialogue box.
• Enter 'appwiz.cpl' and hit Enter to access the Programs and Features window.
• Select 'Turn Windows features on or off' on the left panel.
• Tick the box for '.NET Framework 3.5 (includes .NET 2.0 and 3.0)' and click OK.
• Complete the installation by following the instructions displayed on your screen.
2. Method 2: Applying the DISM Tool
• Start the Command Prompt with administrative privileges.
• Input the command: DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:X:\sources\sxs(Note: Substitute 'X' with the actual drive letter where your Windows installation media is located.)
• Allow the process to finish and then reboot your system.
3. Method 3: Utilising the Windows Update Troubleshooter
• Navigate to the Settings app and select Update & Security.
• Choose Troubleshoot from the menu on the left.
• Select Windows Update followed by Run the troubleshooter.
• Follow the steps provided to complete the troubleshooting.
4. Method 4: Temporarily Disabling Antivirus Software
• Access your antivirus software and deactivate it for a short period.
• Attempt the Office 2021 installation once more.
• After completing the installation, reactivate your antivirus software.
5. Method 5: Executing a Clean Boot
• Press the Windows key + R to open the Run dialogue box.
• Type 'msconfig' and press Enter, opening the System Configuration window.
• Go to the Services tab and select 'Hide all Microsoft services.'
• Click on Disable all and then OK.
• Restart your machine and try the Office 2021 installation again.
• Upon completion, return to the System Configuration and click Enable all.
• Reboot your system once more.