Hey @builtforadulthood ! It sounds like Facebook was connected to a new Manychat account rather than added as a channel inside your existing Instagram account.
Those are different things: each Manychat account has its own billing, even when you access both through the same profile.
First, open the account that already has Instagram and your Pro plan. Then go to Settings > Messenger to add the Facebook Page there, rather than choosing “New account.”
Here’s the official connection guide.
If the Page is already connected to the new account, don’t delete either account yet. Can you confirm whether you now see two separate accounts under Manage accounts, and what happens when you try Settings > Messenger from the original one? A screenshot would help ;)