Error 0x800F081F while installing .NET Framework 3.5 on Windows 10 might be due to corporate WSUS servers not allowing such updates.

Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU in the Windows registry and set UseWUServer to 0 (zero). Restart the Windows Update service (net stop wuauserv & net start wuauserv) and retry installing .NET Framework 3.5. Set UseWUServer back to 1 (one) and restart the Windows Update service.

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