The issue you are running into is because you are trying to create a normal field, but GROUP_CONCAT is an aggregate function so you need to create an aggregate field in your selection view. To do this, go to the '*Group By*' tab and enable 'Use group by'. Then in the '*Output*' tab, click on new, select 'New Aggregate Field' and provide the GROUP_CONCAT function. The [Advanced Operations](https://community.denodo.com/tutorials/browse/basics/3adv5aggregation) tutorial will provide more information.
Hope this helps!