There could be a few places where errors can creep in. The list from csv row is used with the userlist and with the password verification.
Check your google apps script web app. Did you fill in the google spreadsheet and sheet name information correctly, then republish your script to a new version ?
Because the web app uses doGet(e), you can test the script from a browser by building a suitable url (probably good only for testing the getusers function)
You should not need to add dummy data, you should start by making login entries using the app to ensure data compliance.