Encountered the System.MissingMethodException: Method not found with very little else to go on. I noticed as part of the stack trace a reference to an async call on Entity Framework Core Add method.
The error got me thinking about EF Core and I realised I was using SQLite to test an EF Core project. The version of EF Core was 2.26 whereas my SQLite package was targeting 3.1. A quick downgrade of the SQLite package via Nuget package manager and I was motoring again!