Hello,
Thank you for reaching out to us, and I’m sorry to hear about the issue you’re facing with the Content AI in your WordPress shop.
It seems like the Content AI credits might need to be refreshed. Please head over to WordPress Dashboard → Rank Math → General Settings → Content AI and refresh the credits. Here’s a screenshot for reference:

After refreshing the credits, please try accessing the Content AI menu again, and it should work without any issues.
Hope that helps, and please feel free to let us know if you need further assistance with anything else.
Sources:
How to Configure Content AI’s Global Settings » 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.