DropBox API: uploading a file issue

In your /oauth2/authorize URL, I see that you're using the 'response_type=code' flow, which means you're using the OAuth 2 "code flow". In this flow, the code that you get back from that page is an "authorization code", not an "access token". When using the code flow, you need to then exchange the authorization code for an access token by calling /oauth2/token. The /oauth2/token response will contain the access token that you can then use to make actual API calls. (Attempting to use an authorization code in place of an access token to attempt to make actual API calls will result in an 'invalid_access_token' error.) You can find more information on how the OAuth flow works in the OAuth Guide, as well as the authorization documentation. 

And if you're using the official Dropbox .NET SDK anyway, you should use the OAuth helpers it provides, as that will do much of the work for you. The documentation includes examples of how to use those in different contexts, resulting in an access token. There's also a sample web app that implements the OAuth flow as an example.

Also, note that per the documentation, the app authorization page should be accessed at https://www.dropbox.com/oauth2/authorize , not on api.dropbox.com as you have in your post.