The reference assemblies for framework .NETCore, Version=v5.0 were not found

I was chasing the exact same issue but for all I could see, I had the right SDK installed. As it turned out, I had caused the issue myself by placing a global.json in the root directory with the SDK version pinned to 3.1.404. dotnet was honouring the global.json settings and hence could not locate the reference assemblies for 5.0. Removing the global.json fixed the issue for me.

You need to use Visual Studio 2019 16.8 Preview 2 or newer to use .NET 5 Preview 8. .NET 5 Download it here https://docs.microsoft.com/en-us/visualstudio/releases/2019/release-notes-preview

I was getting the same error message. The resolution was to install the latest version of Visual Studio 2019. I had version 16.6 installed on my computer. I needed version 16.8 installed. Once I did the install, the error message went away.

Initial reason for this issue: I had received a VS C# solution from another individual. Apparently there was something in the solution that stated it required some library (.NETFramework v=5.0) which was not in 16.4 but was in 16.8.

The help instructions on the VS error message did not help; there is currently no .NETFramework, version=v5.0. There is only a .NET Core Version=5.0 or a .NetFramework Version=v4.8

This gitHub post lead me to the correct soltuion.