Trouble generating (or identifying) the correct token for file upload

I've gone around and around on this and I suspect I'm missing something simple. I'm lost in a maze of StackOverflow, Google, and Box Community searches and my head is spinning.

I'm trying to upload a file to box.com by generating an admin token and client and trying to impersonate another user. If I don't impersonate the user, I receive a permission denied error, which I understand to be because admins cannot manipulate content. If I do impersonate the user, I receive the "Cannot obtain user token based on the enterprise configuration for your app" error message

My code for setting up the tokens & users is:

        var boxJWT = new BoxJWTAuth(BoxConfig.CreateFromJsonString(jsonConfig));        _userId = "users_email_address";

        var adminToken = boxJWT.AdminToken();
        var adminClient = boxJWT.AdminClient(_userId, adminToken);

        var userToken = boxJWT.UserToken(_userId);

It conks out on the creating the userToken.

The upload code that I have works but only as long as I use the "bearer" method with a developer key, but once I try to switch to an API key things don't work.

var client = new RestClient("https://upload.box.com/api/2.0"); var request = new RestRequest("files/content", Method.POST);

        const string folderId = "12345";
        const string apiKey = "...";

        string Headers = string.Format("BoxAuth api_key={0}&auth_token={1}", apiKey, userToken);        request.AddHeader("Authorization", Headers);        request.AddParameter("parent_id", folderId);

        byte[] byteArray = System.IO.File.ReadAllBytes(path);        request.AddFile("filename", byteArray, "dave-send-fixed.txt");

        var responses = client.Execute(request);
        var content = responses.Content; 

How do I bolt these two pieces together and get a working upload?

Is it possible to get the correct token (either admin or user) from this setup and pass along as the BoxAuth parameter? And/or how do I resolve the permissions error above?

Thanks