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:
The "Consolidate" page is super handy. It lists any packages which have inconsistent versions across projects.
Install button in the lower right to bring them all into line, on the latest version. (Or pick a specific version before pressing