Unfortunately, subdomain can’t be submitted for review.
[…] the URL that you provide:
[..]must not have a path (e.g., example.com/path) or subdomain (e.g., subdomain.example.com)
How to enter the URL of your site when you create an AdSense accounthttps://support.google.com/adsense/answer/2784438?hl=en
Before you sign up – Eligibility requirements for AdSensehttps://support.google.com/adsense/answer/9724?hl=en