Hello,
Thank you for reaching out to us about the issue you are facing with accessing Content AI.
It seems that there might be an issue with the active subscription not being recognized by the system. To resolve this, please try refreshing the credits for Content AI. You can do this by heading over to WordPress Dashboard → Rank Math → General Settings → Content AI and refreshing the credits. Here’s a screenshot for reference:

After refreshing the credits, please try accessing Content AI again, and it should work without any issues.
If you encounter any further difficulties or need additional assistance, please feel free to let us know. We are here to help!
Best regards.
Sources:
How to Cancel Your Content AI Free Trial? » 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.