Package interface &1 has no main package

What causes this issue?

Either the client package is stored in a different main package than the
package interface, or the client package is a main package.
In both cases, you can only create use accesses to the package
interfaces of another main package.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.