Access requests and invitations Link Missing in SharePoint 2013

"Access Requests and Invitations" May not be visible of because of 3 Reasons.

1. "Access Requests and Invitations" link under 'Users and Permissions' will not appear until someone actually sends the requests to join the site. 

http://devsite.com/sites/SP2013/Access%20Requests/pendingreq.asx

try to append /Access%20Requests/pendingreq.asx to your site and check if you are getting page not found error.

If so then send Access request to the site, then it will fix the issue.

All the Access requests and invitations are stored in a hidden list called "Access Requests" and has three views 

1. Guest user invitations (Access Requests/pendinginv.aspx)
2. History (Access Requests/oldreq.aspx)
3. Pending Requests (Access Requests/pendingreq.aspx

  2 .  Allow Access Requests is not enabled

  Site Actions -> Site Settings->Site Permissions ->Access Request Settings

Enable - Allow Access Requests and type mail id to receive the access requests and click on Ok .

  3.   Outgoing email Settings are not configured in Central Administration