If you are in an environment without internet access (or Windows Update fails), you can install .NET 3.5 SP1 directly from a Windows 11 installation ISO or USB drive.
Press the button and type "Turn Windows features on or off" . microsoft .net framework 3.5 sp1 windows 11
For most users on standard versions of Windows 11 (25H2 and earlier), the easiest way to install it is through the built-in "Windows Features" menu: Install .NET Framework 3.5 on Windows 10 - Microsoft Learn If you are in an environment without internet
This is the easiest method for most users as it uses Windows Update to fetch the necessary files. microsoft .net framework 3.5 sp1 windows 11
To install .NET Framework 3.5 SP1 on Windows 11:
Method 1: The Standard Online Method (Versions 25H2 and earlier)
dism /online /enable-feature /featurename:NetFx3 /norestart