Sorry for the late reply.
I somehow mixed conceptual entities (and relationship) with physical ones. I tried to change each of them from conceptual to physical. But it did not work and i still have the same error. So, I exported my ERD in xml format, then re-imported it and it works now.
Thanks for the help.
Bu I have another question, is it possible to reanme quickly some column name from abcDef to abc_def ?