Visual Studio can't resolve Nuget dependencies after reinstalling operating system - nuget

Question

I recently reinstalled my Windows 10 OS, and now Nuget packages are "not compatible". After an explanation, I will be asking, how do I resolve this problem.
Lead up
Due to an unrelated problem, I had to reinstall my operating system. In the course of that, I reinstalled Visual Studio 2019. I cloned my repository (https://github.com/PaulSchrum/CogoDN), then opened it locally with VS.
Now when I build, I get several Nuget errors and warnings. These Nuget dependencies have been present in my code since December 2019, and never caused me a problem until this month (July 2020) associated with the reinstallation of VS.
Question: How do I resolve this problem?
Other Details
I am targeting .Net Core 3.1, but it happens no matter which version of .Net Core I target.
The main error seems to be of the pattern:
NU1202 Package System.Drawing.Common 4.5.0 is not compatible with netcoreapp3.1 (.NETCoreApp,Version=v3.1).
This is happening for System.Drawing.Common, netDxf.netstandard, and packages necessary for Microsoft Tests. Specifically, it can't recognize using Microsoft.VisualStudio.TestTools.UnitTesting; (namespace 'VisualStudio' does not exist), and every attribute ([TestClass], [TestMethod]) now is a syntax error.
What have I already tried
Various combinations of reestablishing the Nuget packages. This includes at least twice clearing the Nuget cache and installing the dependencies again. I also followed the advice here: https://stackoverflow.com/a/63108317/1339950 (Item 3) Close VS, Delete global NuGet.config, start VS and let it rebuild Nuget.config.
None of these work.
One last note: I have tried so many things and got into various different states of not working that my description may have inadvertently conflated a few details.

Answer 1

Visual Studio can't resolve Nuget dependencies after reinstalling
operating system
It is quite strange and in my side, I can also build the project without any errors.(I download the project from your github link and then open it with VS0219----build well).
So I think your Net Core SDK or VS IDE have broken.
1), please try to install the latest Net Core 3.1 Developer SDK.
Or you could open Programs and Features from Control Panel, then right-click on every net core sdk-->Change-->Repair
2) run sfc /scannow to scan your os and you can refer to this link for more steps.
3), try to do a repair in VS Installer. Or if your VS2019 is not the latest, please update it to the latest version.
4) try to create a new net core project and then install these nuget packages to check if the issue still persists.