Have you added the SQLite package to all your projects? You need to add it to the app projects as well as the PCLs.
This is what the error is telling you. It uses a trick called 'bait and switch' whereby it installs a dummy assembly in the PCL with a particular interface, and in the Android and iOS apps it installs a platform specific assembly with an identical interface that actually does the interactions with the database. The idea is you code against the PCL, but at run time the platform specific version of the library is used. You can read more about this pattern at https://web.archive.org/web/20161209075521/http://log.paulbetts.org/the-bait-and-switch-pcl-trick/
Essentially though, this error is telling you that you haven't installed the NuGet package in your platform-specific projects.