I was receiving error code 0x800f081f when trying to install .NET framework 3.5 to work on a legacy project this morning. I would check the box inside the Programs and Features program, attempt to install the feature, but it would fail.
Found out from my company’s IT that Microsoft InTune can prevent it from being installed. A simple
C:\Program Files\Microsoft\OnlineManagement\ProvisioningUtil.exe /UninstallAgents /MicrosoftIntune
executed from the command line silently uninstalls the tool. Restart the computer then install 3.5 successfully!