This article explains the QuickBooks Desktop SDK limitation regarding editing Group Item Descriptions.

Resolution

Modifications to Group Item Descriptions are not allowed as per the QuickBooks SDK. This is the case even when attempting to modify the group item description within the Invoice Line Item grid on the Invoice screen. So even though you’re only modifying the description for that one particular invoice, not to the Item table, it is still not allowed. However, other items, and even child item descriptions can be modified.

For example:

• My Group Item “This description cannot be modified”
• >>Child Item Line 1 “modify this description!”
• >>Child Item Line 2 “go ahead, modify me too!!”