InvalidOperationException: Key type not specified. Microsoft.AspNetCore.ApiAuthorization.IdentityServer.ConfigureSigningCredentials.LoadKey()

As you already answered - The solution is to add the following settings into appsettings.json, within the "IdentityServer" brackets. This followed the "Clients" element:

For File

 "IdentityServer": {
    "Key": {
      "Type": "File",
      "FilePath": "C:\cert.pfx",
      "Password": "password123"
    }
  }

For Development (don't use in production obviously)

 "IdentityServer": {
    "Key": {
      "Type": "Development"
    }
  }

For Store

"IdentityServer": {
  "Key": {
    "Type": "Store",
    "StoreName": "My",
    "StoreLocation": "CurrentUser",
    "Name": "CN=MyApplication"
  }
}

link

The solution is to add the following settings into appsettings.json, within the "IdentityServer" brackets. This followed the "Clients" element:

  "IdentityServer": { 
    "Key": {
      "Type": "Store",
      "StoreName": "My",
      "StoreLocation": "CurrentUser",
      "Name": "CN=**WHATEVER NAME YOU USED AS THE DISTINGUISHED SUBJECT FOR YOUR CERT**"
    }

This follows the instructions here. Note, that this was accomplished with a self-signed certificate created with OpenSSL following this post.