At least one item cannot be processed

What causes this issue?

You allocated items to an invoice and want to process these in
conventional Invoice Verification. Conventional Invoice Verification
cannot, however, process the following purchasing document items:
Items in an invoicing plan
Items in a blanket purchase order
Returns items
Items assigned on a higher or lower level
Service-based items

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

The items which the system cannot process are not allocated to the
invoice.

How to fix this error?

Check if you can continue processing the items selected by the system.
If not, process this invoice in Logistics Invoice Verification.

Error message extract from SAP system. Copyright SAP SE.