If you cannot get a localhost link to work on a URL Redirect in Sandbox here’s what you can do:
You can use any live working link: https://example.com
If you do still want to test a local host workflow for any complete, cancel, or error URL redirects, then you can use ngrok .
You can learn more about the URLs here:
https://community.rapyd.net/t/complete-cancel-and-error-urls
Here are some other examples that use ngrok.