Select a class as a standard class

What causes this issue?

You have allocated an object to more than one class, but have not
selected one of the classes as <DS:GLOS.standard_class>standard
class</>.

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?

Select one of the classes to which the object is allocated as standard
class.

Error message extract from SAP system. Copyright SAP SE.