Dhamodharan,
What generally is happening with a "no collection" message is that
Struts is OK with the syntax of your tags, but when it tries to execute the methods on the beans, it does't find a collection or array as it expects to find.
In the case of your tags, it expects to find a bean named wfdc_RObj in some scope (request, session, etc.) and when it calls the method getWfButtonNames() it expects to find either an array or collection of objects. Similarly, for each object in the collection returned by the getWfButtonNames() method, it executes the method getWfInboxRoutes() and expects to find a collection or array of objects.
Check your data, and verify that there are indeed non-null collections or arrays in these objects.
If these elements could sometimes be null, you will have to surround your iterate tags with <logic:notEmpty> tags.