Projects within a solution use different versions of the same nuget package, consolidate them!

Right click on the solution and choose

Manage NuGet Packages for Solution...

This brings you to a page with tabs: Browse, Installed, Updates and Consolidate

The "Consolidate" page is super handy. It lists any packages which have inconsistent versions across projects.

Click the Install button in the lower right to bring them all into line, on the latest version. (Or pick a specific version before pressing Install)