Had a few instances of installing specific business software or older programs that get a prompt in Windows 10 that .Net 3.5 will be required. They'll either try and add it and fail or you'll attempt to manually install it from the Windows Features section of Control Panel > Programs.
Often you get error 0x800F081F saying that the source files could not be found.
We've regularly seen this in a Windows Small Business Server environment, eg SBS 2008 or SBS 2011. The key fact here is that there's an on-site WSUS server providing local Update Services. Unfortunately it doesn't have the download available for .NET 3.5 for Windows 10 Pro.
There are several recommended fixes involving using a Windows 10 USB installer or downloading the Windows 10 ISO - not always practical on a remote support session - and they don't always seem to work. The best fix and the simplest option we've found to be effective is s asmall registry change, which tells Windows to contact Microsoft directly for Features and Repairs, and not use WSUS.
which should let you Edit Group Policy, see image above.
Look in Computer Configuration > Administrative Templates > System, for the setting 'Specify settings for optional component installation and component repair'
Turn it to Enabled and tick the box 'Download repair content and optional features directly from Windows Updates instead of Windows Server Updates Services (WSUS)'
You might need to start a CMD prompt (use Window-R again) and type 'gpupdate /force' to get the new setting to be applied immediately.