(On Windows Server)
Good news: .NET 3.5 SP1 runs side-by-side with newer .NET versions (4.x, 5+). You don’t have to uninstall anything. .net 3.5 sp1 offline installer
This usually means the installation source is missing or corrupt. Ensure your Windows image is healthy. If using the offline installer, try re-downloading the file to ensure it isn't corrupted. (On Windows Server) Good news: