Hi John,
Thanks for reaching out. The API Center option is not available for the test manager account in the Google Ads UI. Instead it exists in the production manager account. Also the test manager account is shown as closed by default. This is normal so you don’t have to worry about it. You can start using the API if you have a Developer Token and OAuth2 credentials. Please refer to this documentation for more details.
Please let me know if you have any further questions or concerns.
Thanks and regards,
Xiaoming, Google Ads API Team