Group type & has no unique Main Inventory Group

What causes this issue?

The <DS:DE.ADPM_MAINWHG>Main inventory group indicator</> has not been
activated.

System Response

The system is unable to save your entry.

How to fix this error?

Activate the <LS>Main inventory group</> indicator for the new entry.

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.