Mandatory custom field &1 not filled in document header

What causes this issue?

The mandatory custom field &V1& has no value in the sales document
header.

System Response

The sales document cannot be saved in this state.

How to fix this error?

Go to the sales document header details, for example via menu path 'Goto
-> Header -> Sales'.
Then go to the 'Custom Fields' tab and fill in a value for the custom
field &V1&.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.