Hello,
Thank you for reaching out to us. We apologize for the inconvenience you are facing with the Content AI subscription.
To resolve this issue, please ensure that you are logged in to your WordPress website using the same account that you used to purchase the Content AI subscription. Once logged in, please head over to your WordPress Dashboard → Rank Math → General Settings → Content AI and refresh the credits. Here’s a screenshot for reference:

After refreshing the credits, you should be able to use the Content AI plugin without any issues.
If you encounter any further issues or need additional assistance, please do not hesitate to let us know.
We appreciate your patience and understanding.
Sources:
How to Restore Missing Content AI Credits » Rank Math
Content AI – Your Personal AI Assistant by Rank Math
This is an automated reply from Rank Math Support Bot. If you need further assistance, please reply to this topic and a human support agent will reply to you shortly.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.