Capturing Customer Inventory using BLOCK Type "customer_inventory"

Summary

Customer Inventory is not being returned using the BLOCK Type "customer_inventory"

Content

Hello,
I am trying to return a list of the inventory in the customer pool. I haven't been able to return any information in the messages using the following

Subject:
TestBlock

Pattern:
The inventory that remains on WO# {activity_number} is:
<BLOCK type="customer_inventory" >
{inventory_serial_number}
</BLOCK>

The block type is showing up blank (see attached screenshot: test_block.png)

Is there something wrong with the syntax I am using?

Version

19B