Hello,
Thank you for your query and we are so sorry about the trouble this must have caused.
Please make sure to install the Rank Math plugin at the Network level and only activate it on the individual site. You may refer to this guide on how to properly install Rank Math on a multi-site environment: https://rankmath.com/kb/installing-rank-math-on-a-multisite-environment/
Then, reconnect your Rank Math plugin to your Rank Math account. To do that, navigate to your WordPress admin area > Rank Math > Dashboard > Help, disconnect and reconnect your Rank Math account.
Please refer to the screenshots below for your reference:
https://i.rankmath.com/ZvEfVb
https://i.rankmath.com/OZSwOw
Once done, please go back to using Rank Math and it should work without any issues.
Hope that helps and please do not hesitate to let us know if you need my assistance with anything else.