Hello,
Thank you for contacting Rank Math and bringing your concern to our attention. I’m sorry for the delay and for any inconvenience this issue may have caused you.
From the error message, it seems that you’re using the Rank Math PRO version on your website. If you’re still unable to login into your WordPress dashboard, please deactivate Rank Math temporarily by renaming the seo-by-rank-math-pro folder in your website’s root directory – /wp-content/plugins/
No worries, all your configurations will not be affected by deactivating/reactivating Rank Math temporarily. They are saved in your database so it should be fine.
1. Ensure that you are logged in to your paid account:
https://rankmath.com/my-account/edit-account/

2. Disconnect and reconnect your account from inside WordPress:

Please install the Rank Math PRO plugin files if you haven’t already from your registered account from here:
https://rankmath.com/my-account/
and ensure that you are using the latest built:
https://rankmath.com/changelog/
4.You do not need to remove the free version of the plugin. Everything will be ported over to the PRO version.
5. If that does not help, then please add your WordPress and FTP login details by editing the first post on this ticket and adding that information in the designated Sensitive Data section.

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/
Thank you, and we really look forward to helping you.