Error - Pardot Lightning Emails - Select General User or Specific User as the last sender and Select General User or Specific User as the last reply-to user

Error - Pardot Lightning Emails - Select General User or Specific User as the last sender and Select General User or Specific User as the last reply-to user

Issue:
Account Owner is not set or  we need to declare a fallback sender.

Resolution:
May be account owner has not been set for the prospect you're sending. 
Pardot makes us set up a fallback sender, by chance someone on your list does not have an Assigned User. So, click the '+' button next to 'Account Owner' and add the  specific user. You can choose single Pardot User as self as the fallback sender for this email. You can pick any Pardot User in your account. Their names will be available in the dropdown box. If you want to use a fallback sender who is not a Pardot User, you can add a General Sender. This lets you type in a Name and also an email address.