You may receive "Error: A membership on this order is missing a primary member. To complete the order, edit the membership and enter the missing information." in unresolved online sales orders.
To resolve this issue, click "Edit" on the membership line item in the shopping card. From the membership tab, make sure to identify all the members that should be included on the membership (at least one must be listed).
Once you have done this, you can complete the order.