When adding participants to a Survey through the Contact or Custom object, you may encounter an error as shown in the following image. You must configure and add the required external site(s) in the Remote Site Settings.   
Note: This does not apply to Surveys that are being distributed through CoPilot, or when you are adding participants through a .csv file.

add participant error.png

For more information about what is Remote Site Settings and how you can add a new site, refer to the Add External Sites in Remote Site Settings section in the Onboarding Step 1: Install Gainsight & Configure article.