Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
Please check the Replacements settings at WordPress Dashboard > Rank Math > General settings > Images and ensure that values for both Find and Replace are entered and at least one checkbox is checked:

Here’s a link for more information:
https://rankmath.com/kb/image-seo/#replacements
If you are not using the replacements, please click on remove and save the settings.
However, this is a feature of the Rank Math PRO plugin, but the account you have used to open this ticket is the Rank Math free version.
Can you please search and forward a receipt from FastSpring.com so we can locate your account and verify it?
Let us know how it goes. 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.