Hello!
By default, all test accounts are closed because they are no serving accounts. Even though a test account is listed as closed, you can still use it in the API. I took a look at your manager account, and you have two client accounts
(860-520-9335 and 825-131-2129) that were created under the manager account. You creations are working, and you can make API calls under those new child accounts. Make sure to click on the down arrow next to your manager account number, then click the 3 dots, and then select Canceled accounts. Since all test accounts are canceled in the system, you have to select that in order to see them in the UI.
Happy coding,
Nadine, Google Ads API Team

ref:_00D1U1174p._5004Q1zLiHa:ref