I am creating an application using Xamarin.Forms, and in my solution I have three projects that are conflicting with Target Framework types when I run the build.

As shown in the image above, CommonTest is my Testing project, Common is what I am using as a library to store classes used, and SimTools is what I am working out of to create the app.
Currently, when I try to add a reference in SimTools to the Common project to use the classes, I get the error: "Project '..\..\Common\Common.csproj' targets 'net5.0'. It cannot be referenced by a project that targets '.NETStandard,Version=v2.0'."
The SimTools project target framework is .NET Standard 2.0, and the Common and CommonTest projects' target framework is .NET 5.0.
I've tried many combinations of changing the target framework for the Common Projects (changing to .NET Core versions), and the SimTools project (.NET Standard versions).
Additionally, I've referred to Microsoft's documentation regarding what frameworks support one another, and these combinations still show errors when I try to build.
https://docs.microsoft.com/en-us/dotnet/standard/net-standard
What is the minimum target framework version I need to set for SimTools and Common to allow SimTools to have a reference to Common?
