Hi Karol,
Thank you for reaching out. Can you confirm if you are using your own OAuth2 credentials? If so, did you ensure that you selected "Web application" for the Application type when generating the credentials? This is the only type of application that will work with the OAuth2 Playground. Additionally, please try running the OAuth2 Playground in an incognito tab to ensure that you are not logged into a Google account that is not tied to the credentials you generated. If you continue to experience issues, please elaborate on the steps you have taken thus far and I will further investigate.
Regards,
Danica, Google Ads API Team

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