If you’ve checked that the MainWP Child plugin is active on that child site, but you are still getting that error, it is possible that some security layer on your child site side is blocking requests form your MainWP Dashboard.
Do you use some security plugins on the site?
Have you checked the server firewall or any other security layer on the server?
Do you use some cloud-proxy firewall?
If yes, can you please try temporarily deactivating it and see if that helps?