if you are using Google chrome then you have to do the following setting:
1. Go to google chrome setting -> Advance setting-> Privacy & security ->Handlers (Enable your handlers).
2. Sign in to your Gmail account and click the handler icon next to the star icon on the right side of the address bar (shown below). And choose Allow from the dialog box and then click the Done button.
3. Make sure your window default apps should be Chrome for Email.
4. From your Anaplan module or dashboard click on email line item.