Error during Gross/Net volume UoM substitution. See long text.

What causes this issue?

During Gross/Net volume UoM substitution an error has occurred
(Function module GROSS_NET_UNIT_CHECK).

System Response

The specified value for the Gross/Net strategy '&V1&' is not valid.
Only the following values are allowed:
G = Substitute always with the corresponding gross volume unit of
measure
N = Substitute always with the corresponding net volume unit of measure
R = A userexit routine evaluates, whether to substitute with
corresponding gross or net volume unit of measure.

How to fix this error?

The following explanation concerns customizing of Gross/net volume unit
of measure substitution in the R/3 system. If you do not have detailed
knowledge in that area or if you do not have the appropriate
authorizations, contact your system administrator.
Check from where the function module GROSS_NET_UNIT_CHECK is called.
The calling modularization unit should ensure:
that the function module is only called with a valid Gross/Net
strategy.

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.