Please define 1 assortment as the main assortment for customer &

What causes this issue?

You should flag one assortment as the main assortment for each
customer. To do this, you assign the assortment the ranking "1".
This enables you to attribute sales volumes to this assortment in
analyses for materials that are contained in more than one assortment.
You should only assign one such assortment per customer.
Other rankings (> 1) are not currently evaluated in the standard
system, but it is possible to define such rankings for the purpose of
analyses for specific projects by defining RIS user exits.
If you do not select an assortment yourself, the choice of assortment
by the system will be arbitrary.

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?

Error message extract from SAP system. Copyright SAP SE.