Microsoft has released .NET 5.0 and it is available with Visual Studio 2019 16.8.3 and later versions. Use the Visual Studio Installer to check what version you have installed and update to the current release. .NET 5.0 was a short-term release and reached end-of-support on May 8, 2022.
SocketTools .NET 10 Build 1260 and later versions include assemblies which target the release version of the .NET 5.0 Framework as part of the base product. Support for .NET 5.0 is only available with SocketTools 10 and later versions. If you have used .NET 5.0 in your projects, it is recommended you switch to .NET 6.0 which is the current long-term support (LTS) release for the .NET platform.