Hi,
I don't believe this is possible whilst you are using a calculated column in your report view. Calculated columns calculate data in real time and they only exist in the view itself, the calculations are not stored anywhere for Workfront to be able to aggregate into an average or sum.
What I would recommend is building a calculated field that is attached to each of your tasks. This way the turnaround time value will be stored for each task and will be available as a field to use in reports. The benefit is that aggregations can be applied to the field (to work out the average) and it will also be available for use in visualisations such as bar charts (which can't be done with a calculated column).
Best Regards,
Rich.