An Enumeration is always primitive.
Enumerations cannot be broken down and therefore should be always primitive.
In Modelio Enumerations are primitive by default. Only a module or a macro can modify this state.
Contact the developer of the module or the macro which created or modified the Enumeration, and ask him to fix its code.
If you are the developer, call setIsElementary(true)on the enumeration, and check that you are not calling setIsElementary(false)on it.