Issue: VS402913: The release definition cannot be saved because the environment references a service endpoint that cannot be found, or that you do not have permission to access
Solution : The issue is due to the fact that Release Definition is referencing an service end point that has been deleted or when you didn’t have the Permission to access the end point.
If you get an error like the above one, just contact your Project or Org Administrator (Project Collection Administrator) to provide you the required access or ask them to configure the end point.