Hello,
Thank you for contacting Rank Math, and sorry for any inconvenience caused.
Please ensure that there are no server restrictions on that file by contacting your hosting provider first and asking them to check that.
If that doesn’t help, please share your WordPress admin and FTP access in the sensitive data section.
If you are able to edit the robots.txt, the content should look similar to this:
User-agent: *
Disallow: /wp-admin/
Allow: /wp-admin/admin-ajax.php
Sitemap: https://yourdomain.com/sitemap_index.xml
I hope this helps. Looking forward to helping you.
Hello,
I have updated the sensitive data as requested. Can you please check further?
Thank you.
Hello,
I have logged in to your site and was able to edit the robots.txt from Rank Math > General Settings.
I have updated your robots.txt

Result:

I hope this helps. Looking forward to helping you.
Hello,
We are super happy that your issue is resolved. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.