The line item number &1 in the text field &2 is not numeric

What causes this issue?

For FIFO relevant movements the reference for the line item to the
incoming Nota Fiscal must be inserted in a 4 digit numeric format

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.