Hey Noah,
I recently purchased EF and brought it into my Unity 2020.1 project. It's able to login with my auth token, however, when I try to "Create Board" it gets an ArgumentException: JSON parse error: Invalid value.
I put a debug.log in Trello.cs at 300 "Debug.Log(resp.Text)" and it returns with "invalid token".
Could you give the asset a try in Unity 2020.1?