For one of the site we given site owner email address in “Access request settings” , if anyone trying to access the site who don’t have permissions they will request for access to that site email will come to site owner.
When site owner is trying to approve the request he was getting the error “Sending Approval Request Approval Failed” as per the below screen shot.
Navigate to “permsetup.aspx” page in the same site, URL looks like
Select the radio button option “Use an existing group”
In owners Group select the current owners Group name available in the current site and click on Ok.
Now you can able to approve the requests.