Hi Karine,
I'm not clear on your issue.
Is your "Isolation Trace" widget a custom widget? or is it really a Geoprocessing (GP) widget that is referencing a GP service that performance an "isolation trace"?
In the latter case, are you saying the GP service is not working at all in the GP widget in Web AppBuilder? or something else?
The error log you attached seems to be from ArcSDE and indicates that the geometric network datasets you're trying to create already exist.
Is the "isolation trace" model you created working properly in ArcMap? And then you were able to successfully publish its execution plan to create a GP service?