Hi Tamas,
Thank you for reaching out.
Regarding your concern about the API center, test manager accounts do not have an API center page in the UI. This page is only available in the production manager accounts.
Moving forward with your issue on client accounts not showing in your test manager account, test client accounts are shown as "Cancelled accounts" in the Google Ads UI. Kindly refer to the attached screenshot on how to show cancelled accounts in your test manager account. After following the steps, you can now navigate to
Accounts on the left menu bar and see your test accounts.
Regards,
Dannison
Google Ads API Team

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