MatGr / assortment grade: material and assortment do not match

What causes this issue?

You can define which material groups are allowed in which assortment
grade for the assortment owner.

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?

Choose a different assortment grade, or expand the master data for the
assortment owner, or use another listing procedure, or skip this error.

Error message extract from SAP system. Copyright SAP SE.