Please enter data for the material group reference material

What causes this issue?

You have defined the material group reference materials as mandatory
entries in the global settings in Customizing.

How to fix this error?

If you do not want material group reference materials to be defined as
mandatory entries, you must reset the corresponding indicator for
<DS:SIMG.W_GE_0100.SIMG_ORAL>General control of master data</> in
Customizing.
INCLUDE ISR_CUSTOMIZING

Error message extract from SAP system. Copyright SAP SE.