This error occurs when an incorrect attribute(%s) is used along with modifiers like Add, Delete, Replace, and so on in a definition.
Check for spelling mistakes and use the correct attribute.
Example
[#Menu: Gateway of Tally]
Add: Items: "My Report": Display : My Report
Since the attribute Items is not a valid attribute with Add modifier for the Menu. So the message error T0013: Could not understand the attribute 'items' for a modifier. appears.
To correct the code, changed the attribute Items to Item.
[#Menu: Gateway of Tally]
Add: Item: "My Report": Display : My Report