Why is Form 1065 not accepting a valid Principal Business Code and will not allow me to efile?

From this year's instructions

Nonstore Retailers
Nonstore retailers sell all types
of merchandise using such
methods as Internet, mail-order
catalogs, interactive television,
or direct sales. These types of
Retailers should select the PBA
associated with their primary
line of products sold.
For example, establishments
primarily selling prescription
and non-prescription drugs,
select PBA code 456110
Pharmacies & Drug Retailers.

Business Codes were revised and updated by the IRS. You will need to find a new category from the updated list. The business code section starts on page 18 of the link below. I understand you are not a Schedule C, but the updated list is available in this publication.

Schedule C