Packaging material types are not uniquely set in Customizing

What causes this issue?

Setting <DS:DE.EXIDV_UNIQUE>unique number assignment</> for the
handling unit identification numbers is only possible if none of the
existing <DS:DE.VHART>packaging material types</> has arbitrary number
assignment.

Procedure for System Administrators

Maintain the <DS:SIMG.SIMG_HUMIDTVTYNR>number assignment of all
packaging material types</>.

Error message extract from SAP system. Copyright SAP SE.