No customer number is defined

What causes this issue?

The customer number assigned by the recycling partner is missing.

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?

If you want to carry out the declaration, you need to add the customer
number assigned by your recycling partner to the declaration data.
Maintain the customer number in the recycling partner maintenance of the
recycling partner &V1& in the company code &V2&.

Error message extract from SAP system. Copyright SAP SE.