If you are trying to complete an email task but are receiving the error message 'Unable to find sender user "system" ' error message:

The reason this error occurs is because that task is assigned to Senta by default, and the default sender of the email is also assigned to Senta, which is not permitted.

To fix this error:

1) Open Settings > Jobs > Job in question

2) Click on Tasks > The send email task in question

3) In the task settings window, click on Email. 

4) Change the default Sender of the email from Assignee to another registered user in the practice:

5) Click OK and then click Save

You should now find that the task will complete automatically, and the email that is sent will be from the user that you chose in the task configuration.