Item not picked yet; conversion not possible

What causes this issue?

A delivery line item has been selected for quantity conversion, but has
not been fully picked yet.

How to fix this error?

Complete the picking process; the picked quantity must equal the
delivery quantity before quantity conversion can be made. The quantities
will then be adjusted based on the results from the Quantity Conversion
Interface (QCI).

Error message extract from SAP system. Copyright SAP SE.