Hello,
We apologize for the issue you are facing with the invalid API key message while trying to use Content AI. This error typically occurs when the API key is incorrect or has not been copied correctly. Here are some steps you can follow to resolve this:
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Check Your API Key: Ensure that you have copied the correct API key from your Bing Webmaster Tools account. You can find it by navigating to Settings → API Access → API Key in your Bing account and clicking the Copy button.

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Add the API Key in Rank Math: In your WordPress admin area, go to Rank Math SEO → Instant Indexing. Under the Settings tab, paste the Bing API Key and click the Save Changes button.
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Avoid Regenerating the API Key: After configuring the settings, make sure not to regenerate the API key in your Bing Webmaster Tools, as this will invalidate the existing key.
If you continue to experience issues, please reply to this ticket with your site URL, and a human agent from our support team will assist you further. Thank you for your patience!
Sources:
How to Fix Failed to Submit Post to the Bing URL Submission API Error » Rank Math
Hello,
We might need to take a closer look at the settings. Please edit the first post on this ticket and include your WordPress & FTP logins in the designated Sensitive Data section.
Please do take a complete backup of your website before sharing the information with us.

It is completely secure and only our support staff has access to that section. If you want, you can use the below plugin to generate a temporary login URL to your website and share that with us instead:
https://wordpress.org/plugins/temporary-login-without-password/
You can use the above plugin in conjunction with the WP Security Audit Log to monitor what changes our staff might make on your website (if any):
https://wordpress.org/plugins/wp-security-audit-log/
Looking forward to helping you.
Thank you.
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.