No CusPlntNo. is maintained in T001W for plant &

What causes this issue?

In IS-Retail, you can only maintain plants with reference to a
customer number in the plant table (T001W-KUNNR).

How to fix this error?

When creating a plant with the IS-Retail transaction (WR01), this
customer number is automatically updated.
If these are plants that were created within standard views, you
should try to create a customer number with the appropriate account
group (or plant profile) with the standard function from SD
(Logistics -> Sales/distribution) and add this in the field
T001W-KUNNR using the 'Table Maintenance T001W' function.

Error message extract from SAP system. Copyright SAP SE.