You are getting this error because your Instagram has "Access to Messages" disabled. Before going through the connection process again, you must allow Facebook to access your Instagram messages.
1.1 To check and allow Facebook to access your Instagram messages via app
Go to your Instagram profile.
Select Settings and activity.
Under How others can interact with you, select Messages and story replies.
Tap Message requests.
Under Connected tools, enable Allow access to messages.
3.2 To check and allow Facebook to access your Instagram messages via desktop
From here, toggle on the option Allow access to Instagram messages in Inbox.
Log in to your Instagram account.
1.2 Allowing Facebook Access to Messages
Go to your Instagram profile.
Select Settings and Activity.
Under How others can interact with you, select Messages and story replies.
Tap Message requests.
Under Connected tools, enable Allow access to messages.
