Hello,
Thank you for your patience.
We’ve checked your account, and the Content AI subscription is already applied.
Please remove your website from your Rank Math account dashboard: https://rankmath.com/my-account/websites/
Once done, please ensure that your Rank Math plugin is already updated to the latest version:
https://rankmath.com/changelog/free
If the issue persists, please follow these steps:
1 . Please clear the transients at Rank Math > Status & Tools > Database Tools > Remove transients

2 . After that, you may try disconnecting and re-connecting Rank Math to see if that works for you.
You can disconnect your account by navigating to WordPress Dashboard > Rank Math > Dashboard > Help and then simply disconnect the account by clicking on Disconnect Account.

3 . Once you’ve disconnected your Rank Math account, you can connect it again in the same settings. To connect your correct account, click on Connect Now.

4 . Once you click on Connect Now, please ensure that you’ve selected the correct account and then click on OK, Activate Now.

5 . Once all the above steps are done you might need to refresh the credits again this time to start using Content AI.
Please head over to WordPress Dashboard → Rank Math → General Settings → Content AI and refresh the credits. Here’s a screenshot for reference:

Let us know how this goes.