I had an odd problem with incompatible projects today, and I am posting this in case it might help someone who has run into a similar issue.
Up until yesterday morning (5/19/2018) I was able to open my nopCommerce 4.00 solution in Visual Studio on my developer machine and view the source code, build it (except the Nop.Plugin.Payments.Square project, but since we are not planning to use Square, I simply excluded it from the build), and run it. Today, however, when I tried to open the solution, all projects were flagged as incompatible.
I am not aware of any changes or updates on the machine since yesterday morning, and I could not find any issues with system requirements or .NET configuration.
Eventually I hit upon trying to reload the projects in Visual Studio (right-click each solution folder and select Reload), and that ended up resolving the problem.
Thank you!
Lena Nilsson