Help with this error in xamarin Forms

Entering the properties of your project you can change the Target Framework Version to v7.1 that your compiler requires.

You are using a library that is Android v7.1 but your project is made for android v6.0. You have to upload the version to your project entering the configuration of the same and if you do not have it, download it by the sdk manager. Otherwise you will have to search the library for v6.0.