Part of your assembly already exists in TC or you don't know? Have you tried the Data Prep utility steps before using AtT-Interative? If not, start over with those steps. Also try full AtT instead. It is slower but will give you better error messages.
I don't know why I never looked at the message on the left handside of the page in Add To Teamcenter.
I had 2 files with the same name but the file extension was different (.asm & .par) and it seems this is not allowed in Teamcenter.
That is correct. You can only have one model per Item Revision. You can have as many drafts as you like but only one model. You'll have to put one of them in with a different item id. Go back to the data prep tools to do this and update links before running AtT again.
The parts are now in TC.
I understand about not allowing the 2 models with the same name, but the file extension should be able to differentiate between the two.
Thanks for your help pointing me in the right direction.