Unloading broken assembly: Google API

Hi, I'm working on a project developed in Unity 2018.3.0f2 for mobile.
I'm trying to use Google API in order to Read/Write some data in a Google Sheet.
I've downloaded the nuget packages of the various part composing the API, converted the extension to .zip and extracted the .dll file in lib/neystandard2.0. I moved all the .dll files in a folder I created in Assets named Plugins. The files are:
- Google.Apis.Auth.dll - Google.Apis.Core.dll - Google.Apis.dll - Google.Apis.Oauth2.v2.dll - Google.Apis.Sheets.v4.dll
I set the API compatibility level to .NET standard 2.0 in the player prefs.
In visual studio 2017 everything look fine, however in the editor I'm obtaining the messages:
"Unloading broken assembly Assets/Plugins/Google.Apis.Auth.dll, this assembly can cause crashes in the runtime"
"Unloading broken assembly Assets/Plugins/Google.Apis.Core.dll, this assembly can cause crashes in the runtime"
How can I fix it?