Yes, using /2/files/list_folder[/continue] is the right way to list the files in an account. To list every file, you would set the 'path' parameter to the empty string "" (not "/"), and set 'recursive' to true.
The error you're getting indicates that the call failed because the HTTPS request body was empty, instead of containing the expected parameters. I see from your code that you are attempting to set the body via the 'SetBody' method, but that's apparently not actually getting sent to the server.
I can't provide support for the Appeon PowerBuilder environment or OAuthClient/OAuthRequest/ResourceResponse classes themselves as they aren't made by Dropbox though, so I recommend referring to the documentation for those for information on how to configure/debug HTTPS requests like this.