Olivier,
I’ve engaged Elegant Themes (Divi) and they were able to look into the issue further. It looks like there seems to be an issue with the initial Rank Math setup where it asked if I have a Google Account (which I have set up with my main domain), then the fact that the site I am working on is on a subdomain.
They feel that when I migrate this site to the main domain this issue should go away and if it doesn’t they provided some suggested steps to solve it.
Thanks for reaching out.