Hi all,
Thank you for reaching out to us. I’ve attempted to create a test client account under a test MCC account and was able to by using this example from the client libraries. Because this looks to be an issue with the Google Ads UI, could you please reach out to the Google Ads Product Support Team here, as they are better equipped to assist you with UI related concerns. Let me know if you have further questions about AdWords / Google Ads API.
Thank you,
Bryan, Google Ads API Team
click the blue "+" button in Accounts, enter in some details and click "Save and Continue". This should create a test account under your manager account.