The message "Cannot modify attachment level attribute" when attempting to change reference levels in the Level Manager.
This message appears in the Message Center
Go to Workspace > Configuration > Levels and look at the setting for the variable labeled "Control Level Attributes Edit" (if looking for the actual variable name, it is MS_LEVEL_EDIT_ATTRIBUTE_LIST).
If this variable is blank (NOT undefined), then it will prevent you from editing any attributes.
Either delete this variable definition, or use the list found in the description of the variable to add attributes that you want to be able to edit. Deleting the variable value so that it reads as [Undefined] is the default setup for this variable.