You can install .NET Framework 4.8 on:
Windows 10 October 2018 Update (version 1809)
Windows 10 April 2018 Update (version 1803)
Windows 10 Fall Creators Update (version 1709)
Windows 10 Creators Update (version 1703)
Windows 10 Anniversary Update (version 1607)
Windows Server 2019
Windows Server, version 1809
Windows Server, version 1803
Windows Server 2016
The .NET Framework 4.8 is not supported on:
Windows 10 1507
Windows 10 1511
If you're using Windows 10 1507 or 1511 and you want to install .NET Framework 4.8, you first need to upgrade to a later Windows 10 version.
What is your OS build?