Error:

Error Code: 21919028

Short Message: Portions of this listing cannot be revised if the item has bid or active Best Offers or is ending in 12 hours.
Long Message: Portions of this listing cannot be revised if the item has bid or active Best Offers or is ending in 12 hours.


Resolution:
This error can be caused by a number of fields which are restricted from being edited when one of the above conditions is met.


The most common cause of this error is when someone attempts (
accidentally or intentionally) to disable Best Offer on an item while it has pending offers or when it is ending soon.  
To resolve this, you must turn Best Offer to True then Update Item.

However, this error can be caused by similar issues with other fields - to see a full list of restrictions eBay has on revising listings (especially within the last 12 hours of the listing duration) visit: 
http://pages.ebay.com/help/sell/revising_restrictions.html