This article outlines the requirements for connecting a Facebook account, including that you will need to be either an admin or editor in each account you'd like to connect.
The only way I was ultimately able to connect (and then re-connect) FB and Instagram accounts was by signing in again using a New Private / Incognito Window. Then all browser extensions and plug-ins are disabled. Good luck!
This article outlines the requirements for connecting a Facebook account, including that you will need to be either an admin or editor in each account you'd like to connect.