You must select the requester Email Address on Form Workflow Plus sidebar Advanced tab.
The selected Email Address is used to inform the Requester the status of their workflow request at the end of each Approval Step. If the workflow request needs to go through to another Approval Step the requester is notified and should wait until all Approval Steps are completed.
Check Send back to Requester to allow the Reviewer to send a workflow request back to the Requester for revision
To be able to select Email Address and allow Send Back to Requester , please ensure you choose Collect email addresses on your Google Form