In order for an account to be set as the practitioner when creating an invoice, the user account must be set as visible from within the Users page.
To do this first navigate to the Administration dashboard and click "Users".
An image that has been edited to show the location of the Users button.Find the account that you would like to edit and tick the box shown in the Visible column by clicking on it. The user can now be set as the practitioner when creating an invoice.
A screenshot of the Users page showing the location of the Visible column.