The .NET Framework 3.5 is essential for running many older desktop applications on modern operating systems. While Windows 10 and Windows 11 ship with newer versions like .NET 4.8 pre-installed, they do not enable version 3.5 by default. Attempting to install it online can sometimes fail due to network restrictions, Windows Update errors, or lack of internet access.
Locate at the top of the list. Check the box next to it. Windows Update errors