Solution
An Enum can only be changed in Edit Mode. If you try to change it in runtime, you will get the error.
You can't edit an Enum in runtime. An alternative is to use VI Server; you can refer the example from the forum
Programmatically Add Elements to Enum / Ring.
Another alternative is to use a text ring control instead. You can use the ring control to programmatically read and write the strings through the Strings property.