Starting with , Microsoft has changed how this framework is delivered, making it a standalone installer rather than a built-in Windows component. Why You Need .NET Framework 3.5 SP1
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 microsoft .net framework 3.5 sp1 windows 11