Microsoft Win 10 Pro .NET 3.5 unable to turn on or download

Question :

I have a problem with a new windows 10 version 1709 build 16299.125.

I require Microsoft.net 3.5 to install my Sage Accounting software so you think programs and features > turn on windows features > check the 

.NET Framework 3.5 (includes. NET 2.0 and 3.0) and hit ok.

This leads you to windows update > download files. you wait > error code 0x800F081F.

This is where the trail runs dark as we get this from Support @ Microsoft.

Error code 0x800F081F

This error code can occur when an alternative installation source is specified and one of the following conditions is true:

• The location that is specified by the path does not contain the files that are required to install the feature.
• The user who tries to install the feature does not have at least READ access to the location and to the files.
• The set of installation files is corrupted, incomplete, or invalid for the version of Windows that you are running.

To fix this problem, make sure that the full path of the source is correct (x:\sources\sxs) and that you have at least Read access to the location. To do this, try to access the source directly from the affected computer. Verify that the installation source contains a valid and complete set of files. If the problem persists, try to use a different installation source.

Anser : 
Open Notepad  Copy and paste the following into it:

@echo off
Title .NET Framework 3.5 Offline Installer
for %%I in (D E F G H I J K L M N O P Q R S T U V W X Y Z) do if exist "%%I:\\sources\install.wim" set setupdrv=%%I
if defined setupdrv (
echo Found drive %setupdrv%
echo Installing .NET Framework 3.5...
Dism /online /enable-feature /featurename:NetFX3 /All /Source:%setupdrv%:\sources\sxs /LimitAccess
echo.
echo .NET Framework 3.5 should be installed
echo.
) else (
echo No installation media found!
echo Insert DVD or USB flash drive and run this file once again. 
echo.
)
pause

Save it as as framefx.cmd

Right click framefx.cmd

Click Run as administrator

The file will install .NET Framework 3.5 automatically after finding the drive letter of your Windows 10 installation media.

If media not found insert Win10 Usb or Dvd drive and run as administrator again.

If there is an issue in regards to your install media, download the Windows 10 ISO file then double click it to mount it.