The most common cause and resolution of this error message is documented in the following troubleshooting article:
https://developer.xamarin.com/guides/cross-platform/troubleshooting/questions/nuget-packages-missing/
Essentially, certain configurations of NuGet hard code the package imports into the .csproj file itself. The most frequent time I've seen this behavior is when a Xamarin.Forms app is created in Visual Studio and migrated over to Xamarin Studio; however it can happen in other cases.
The offending lines in the .csproj file look something like this:
<Reference Include="Xamarin.Forms.Maps">
<HintPath>..\..\packages\Xamarin.Forms.Maps.1.3.1.6296\lib\portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.Maps.dll</HintPath>
</Reference>
<Import Project="..\..\packages\Xamarin.Forms.1.3.1.6296\build\portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets" Condition="Exists('..\..\packages\Xamarin.Forms.1.3.1.6296\build\portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets')" />
<Error Condition="!Exists('..\..\packages\Xamarin.Forms.1.3.1.6296\build\portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets')" Text="$([System.String]::Format('$(ErrorText)', '..\..\packages\Xamarin.Forms.1.3.1.6296\build\portable-win+net45+wp80+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets'))" />
Deleting these lines (or commenting them out) generally resolves the issue.