Solution:
From learning.sap.com, the valid invoicing types used for a Blanket Purchase Order (BPO) are:
* A. PO-based Invoice - Specifically for item-level BPOs, you can create a standard invoice using the purchase order (via PO-Flip). SAP Learning
* D. Contract-based Invoice - For other BPO types (such as supplier-, catalog-, or commodity-level BPOs), you must enter a non-PO invoice that references the contract/BPO ID - which is a contract-based invoice. SAP Learning