Is there a way in Nintex where a manager can view his employees pending tasks?
One easy way is to create a view of the List and have the filter set to something similar to below. This is based on a normal custom List, but could be applied to the Task lists. Of course, this is reliant on the Manager field being present and populated and this being a single Task list. :
Afaik not by default, you would have to create your own solution for that.
This is what you need: SharePoint Power Hour Episode 107: Content Search, direct reports tasks - YouTube
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