Material &2 not defined specifically for JIT customer &1

What causes this issue?

Material &V2& has not been defined specifically for customer &V1&.

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?

You have defined several SD delivery schedules for material &v2& and
customer &v1&. To assign the material uniquely, you must specify the
unloading point upon receipt of the sequenced JIT call. You should
define this in the SD delivery schedule.

Error message extract from SAP system. Copyright SAP SE.