Are you not better off using SQLite.net.pcl (Frank Krueger) rather than SQLite.net-PCL (OysteinKrog)?
SQLite.net-PCL isn't being maintained anymore see the top of the README here. The OysteinKrog version was a fork of the Frank Krueger (Original) version, I think due to it not being actively developed years ago but now Frank and others are actively maintaining the project. I would suggest switching to it.
SQLite.net.pcl supports .NET standard, see the release 1.3.0 here
In saying that the warning you are seeing is just letting you know that it has targeted the .NET Framework v4.6.1 version of the NuGet package rather than finding a .NET Standard version. This means it has found a version but the NuGet Package doesn't officially support .NET Standard 2.0 so it could still work but is not tested.