What causes this issue?

When entering a combination of material number and serial number, you
have only specified one of the two numbers.

System Response

If this is an error message, the system expects the two numbers to be
specified.
If this is a warning and you confirm it with ENTER, a selection is
performed that displays all relevant combinations in a selection list.
This selection can take a long time.

How to fix this error?

In the event of an error message, specify both numbers.

Error message extract from SAP system. Copyright SAP SE.