While using a displayname for userpicker field in SubTask it doesnt work

Hi Team,
1. In the Main Issue:
We have a user picker field, and I want the email of the field.
Could you please let me know how to extract the email name of the user picker data?

2. We are currently using a user picker field in the SubTask.
And I want to export the user name.
So when I use

 ${Subtasks[n].'displayname:fieldname'} 

or

${Subtasks[n].displayname:fieldname}

it doesn't work. It doesn't display any data, its blank.

Thanks in Advance

Best Regards,
Prasanjit