Hi Dennis,
Thanks for reaching out. Your test customer account is already created. The test manager account by default is in closed status. If you click on the down arrow next to the test manager account number and on the drop-down list click on the three dots icon and select cancelled accounts. Then click on the Accounts tab on the left hand side you would be able to see all the test customer accounts including the one you just created under this test manager account. You could use the ManagedCustomerService.get() to retrieve the account hierarchy information which also includes all the cancelled accounts. This is a code example for you on how to use this service. Please let us know if you have any further questions.
Thanks and regards,
Xiaoming, Google Ads API Team