MG materials can only be maintained in SAP Retail clients

What causes this issue?

You can only maintain reference and value-only materials if your system
is configured as a <DS:TX.ISR_RETAIL_TEXT>SAP Retail system</>.

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?

The consequences of changing your system configuration to a SAP Retail
configuration are far-reaching.
INCLUDE ISR_VERWALTUNG

Error message extract from SAP system. Copyright SAP SE.