If you encounter the following for the sandbox:
The specified client email does not exist. Your client accounts may not exist because either this is your first time using the sandbox or the sandbox database has been cleaned. Please remove the clientEmail from the request header and call the getClientAccounts method from AccountService to ensure that your client accounts are created and do exist.
Do what they mean😉 !!
1. Run your application once with an empty clientId/clientMail (so use “” in the Java API) … then you will get an error, which is okay
2. Run your app a second time. But then specify the correct clientId (sth. like “firstname.lastname@example.org”) and all should work fine.