This error usually occurs when you click the Send feedback link on the Fortnite tab of the Epic Games launcher, but do not have an application set up to handle mailto links. You can correct this by either downloading and setting up a desktop email client such as Thunderbird, or by configuring your webmail service of choice to be the default mail client for mailto links. The steps required to do this will vary based on your mail client and browser. If you're unsure of how to do this, we recommend contacting your mail client's support team.
If you are receiving this error and are not attempting to send Fortnite feedback, please contact Player Support using the Email Us link on the sidebar of this page. Make sure to include your dxdiag and launcher debug logs.