I’m having this problem too it is very annoying. Have you resolved it? 

This means that the page is already connected to another Manychat account.

Here's some documentation about it: https://help.manychat.com/hc/en-us/articles/14281291094428-Unable-to-connect-Instagram-to-Manychat

Just got word from the Manychat team, here's an updated article on how account ownership works:

https://help.manychat.com/hc/en-us/articles/14281189858332-How-to-manage-ownership-of-your-Manychat-account