If an office user opens a completed appointment in the Calendar, an error message might appear if the user does not have the Invoicing role in their user settings.

To allow an office user to view the Invoice and proceed to the Work Order, go to Settings/Users & Routes, edit the user's property page, and give the user the Invoicing role.

If you do not want the user to have access to Invoices, have the user go to Sales > Work Orders and access Work Orders from there.