The problem is that you are trying to use versions of the packages not compatible with your Target Framework , for this you must go to the project > [project name] properties ... and in the Application section select the required Target :
In your case a version of 7.0 or higher is required to restore and install the packages without problems.
Before downloading a Nuget package, check its dependencies, so you can make sure that packages are installed and that target requires to do so.
You can search for Nuget packages on its official website: link and on the Dependencies tab you will find packages you will need to install to work and what mono android version you will need to specify in your Target .
Note: If at any time you do not see a specific target, you must update your Visual Studio version and install the corresponding SDKs in the visual studio tab, Tools > Android > Android SDK Manager .