Is there a way to notify the sender once the receiver completes a task?

is there a complete button? For exsample, once the receiver completes the task they could click on a complete button to notify the sender

The submit button should do it. If you want to notify the submitter, you have to create an autoresponder email in the form. Here's our guide: https://www.jotform.com/help/26-Creating-a-Form-Autoresponder.

If you would like to get a notification once the form is submitted, you have to create a notifier: https://www.jotform.com/help/25-Setting-Up-Email-Notifications. 

I'm sorry, I think you mean to notify the user after you receive an edit the submission. If so, you can do something like this:

https://www.jotform.com/help/194-How-to-assign-a-status-to-submissions

You have to conditionally send an email based on the completed task: https://www.jotform.com/help/167-How-to-send-to-a-specific-email-address-based-on-a-selection.