I have the same error message as this question, causing my Xamarin Forms project to not deploy. This is because each platform target of my project (iOS, Android and the Shared Platform) is running different versions of Xamarin forms.
The answer in the linked question was:
Solution:
Change in your Core, Droid and IOS project the Xamarin.Forms versions. Make sure that all versions are the same.
How do I change my Xamarin.Forms versions for each of my target platforms? Here is what my navigation menu looks like:


