What causes this issue?

You have entered a combination of material type and valuation area for
the plant that is not defined in the system.

System Response

An entry made up of this combination is normally created automatically
for all valuation areas in table T134M if you execute the function
<DS:TRAN.OMS2>Edit material type</> in Customizing for the
corresponding material type and then save your data.
The absence of an entry can be due to you setting up and assigning an
additional plant to a company code. Although the system then determines
the respective company code automatically, it does not create an entry
in table T134M for the existing material types in this case.

How to fix this error?

Contact your system administrator so that the corresponding combination
can be entered in table T134M.

Error message extract from SAP system. Copyright SAP SE.