Getting assembly reference error (Unity)

[haebichan]

"error CS0246: The type or namespace name 'Message' could not be found (are you missing a using directive or an assembly reference?)"

Can anyone help with this issue? I don't know if this is a Net Framework compatibility issue. I'm trying to integrate Slack API with Unity. The Unity version is 2019, and I've set the API compatibility level to both .NET 4.x and .Net standard 2.0. Both does not seem to work. Thanks!