Solved
Even though the element has been mapped , TC still popup dialog '..because it is not mapped..'.
As the subject title said, I have already mapped the element into the NameMapping.
But when I clicked 'Show object in Name Mapping Editor', the error dialog displayed.
Like the capture:
...
First I would delete the mapping for that object and try again. You may have to go a couple of objects up the tree as well, I have seen this happen when the end object is ok but one of the parents is not right.
