Hello,
Thank you for contacting Rank Math, and sorry for any inconvenience caused.
Please ensure that you have created a Universal Analytics property and not GA4.
https://rankmath.com/kb/using-ga4/
If you have created the Universal Analytics property, reconnect Rank Math to your Google account by going to Rank Math > General settings > Analytics > Reconnect

Once done, you will be able to select the property from the Rank Math Analytics dropdown.
I hope this helps. Looking forward to helping you.
Thanks a lot for your awesome support.
Hello,
We are glad we addressed your concern. 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.