In order for a survey to be successfully sent, a token needs to be attached to the ticket by the Survey app. (This happens automatically behind the scenes within about 5 - 10 seconds of the ticket being created).
If the token is not being added then when the end-user clicks into their survey, they will see the error message: "Survey expired".
There are two things you can check to determine why this is happening.
1. Run app diagnostics.
In the main left bar, under Admin -> Diagnostics, click on Run check like this:
This will check to make sure everything else in the app has been setup correctly. If any errors popup after running diagnostics, simply click to fix the issue.
2. The survey is being sent too quickly.
If you're sending a survey the same instant the ticket is created (or very soon after), then the app has not had the required time it needs to apply the token to the ticket.
To fix this you will need to wait until the token is added, or alter your workflows to allow for this delay. See how the Survey app link and token works for more information.