Customer object &1 &2 cannot be assigned to package &3

What causes this issue?

Object &V1& &V2& is in the name range reserved for customers. The
object cannot be assigned to package &V3&, since it belongs to a
different name range or namespace.

System Response

Editing is terminated.

How to fix this error?

Assign the object to a customer package whose name begins with Y or Z.
If you want to create the object as a test object, you can also assign
it to a local package (1st character = $) or a test package (1st
character = T).

Error message extract from SAP system. Copyright SAP SE.