In Xamarin.Forms Ive found when starting a new Shell project I have come across the error XLS0503. It usually comes about during the building of the menu structure. As you add menuitems and shellcontent to shell, the XLS0503 error will begin to pop up.
Pretty simple, completely clean the solution by right clicking on the solution and click clean. That usually solves it.
But if you are still having issues, you can also delete the bin and obj folders in the solution. All done.