if you want to use validation message from ENUM. Then assign that ENUM to any one of your attribute.
Consider your attribute is ValidationMessage, Assign ENUM to this attribute.
On some condition you might be updating the ValidationMessage attribute using ENUM value.
Consider your ENUM has following messages:
Name is required.
Mobile Number is required.
In your nanoflow, you see user not filled the name field. At that time you want to show validation message : Name is required.
In that case you can update the ValidationMessage attribute with ENUM value
$ValidationMessage = ENUM_ValidationMessages.Name is required.
First Can you check if you proper access to the attribute also check
if ($currentObject/Enum != empty)