Here’s what we found on the Rank Math blog post.
1. Login to your Pinterest account and go to https://in.pinterest.com/settings/claim/ enter your site domain and click claim. In the pop up select the add html tag option, copy the meta tag and add it to Rank math > General settings > Webmasters > Pinterest verification ID field
(here is a screencast for reference: https://i.rankmath.com/pYSeod)
It should take at least a day to verify and claim your site.
2. Take one of your posts URL and follow the instructions here at Rich pins Validator
Hope that helps. If you have any further question(s), please contact the support team at Rank Math pro here.