Hi M. T ,
Welcome to Microsoft Q&A platform and thanks for posting your question here.
As I understand your query, you are trying to make use of except function in dataflow which returns a difference set of one array from another. However, you are getting error since the array values are not getting formatter properly . Please let me know if that is not the case.
Kindly try one of the below expressions to get the result:
Hope it helps. Kindly accept the answer if it's helpful. Thankyou.