Hello,
We apologize for the issue you are facing with adding redirections in Rank Math. It seems that you might be using an older version of the plugin, as the current version may have improvements that could resolve this issue. We recommend updating to the latest version of Rank Math to ensure the best experience and functionality.
If updating does not resolve the problem, it could be due to missing database tables. You can recreate any missing tables by navigating to Rank Math SEO → Status & Tools → Database Tools in your WordPress dashboard. Make sure to enable Advanced Mode if you do not see the Database Tools option. Then, click on Re-create Missing Database Tables.

If the issue persists after trying these steps, please reply to this ticket with your site URL, and a human agent from our support team will assist you further. Thank you for bringing this concern to our attention!
Sources:
Solving Redirection Issues Using Rank Math SEO Plugin » Rank Math
This is an automated reply from Rank Math Support Bot. Your ticket has been assigned to a human agent and we will get back to you shortly.
The Problem still persists…
Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
Assuming you are using the latest version of all the plugins and the themes (please update if not yet), it would seem like a conflict with one of the plugins or the theme you are using.
To determine that, please follow this troubleshooting guide:
https://rankmath.com/kb/check-plugin-conflicts/
Here is a video you can follow as well:
https://www.youtube.com/watch?v=FcIWRhcUP2c
Only the site admin would know and your users won’t be affected by this troubleshooting.
If the issue persists, 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/
Please do take a complete backup of your website before sharing the information with us.
We really look forward to helping you.