Hello,
Thank you for reaching out to us regarding the issue with your Content AI subscription.
To activate your Content AI plan after purchasing the Starter plan, please ensure that you have refreshed the credits. You can do this by going to WordPress Dashboard → Rank Math → General Settings → Content AI and refreshing the credits. Here’s a screenshot for reference:

After refreshing the credits, you should be able to access Content AI without any issues. If you encounter any further difficulties, please feel free to let us know so we can assist you accordingly.
We appreciate your patience and understanding.
Sources:
Content AI Credits and Plans: Everything You Need to Know » 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.