To connect or reconnect a Facebook Page or Instagram business account, you'll need to authenticate through Facebook. In order to connect successfully, you'll need to make sure your personal Facebook profile is an admin on the Facebook Page that you want to connect/reconnect or that is linked to the Instagram channel you want to connect/reconnect.
Head over to Facebook and navigate to the list of Pages you have access to (or click this link). Click on the Page you want to view.
Once you're in your Page, click Settings in the left hand sidebar.
Click Page roles on the left hand side.
You’ll need to see your own personal profile listed as an admin under the "Existing Page roles" section, as shown below.
If you have any other role, then you will not be able to authorize the connection. In this case, you'll need to get in touch with the admin of the Page and ask them to grant you the admin role, or alternatively, have them connect the Facebook Page or Instagram channel to Buffer instead.
It's also important to note that it is not possible to connect a Facebook Page if you are an admin through inheritance from a parent Page. If you see yourself listed on the Page Roles section under a section labeled "Admin (inherited from parent page)", then unfortunately you will not be able to authorize the connection between the Facebook Page and Publish.