Your username on the computer is
NOT your Apple ID. They are two separate things.
Your username is the name that was used when you first started using your computer. As an early step in setting up your computer, macOS asks for your first and last names. (Or maybe it asks for your Apple ID and then gets your first and last names by looking them up in Apple's database.)
If you used the first name "John" and the last name "Smith", the long form of the name would be "John Smith", and the short form would be "johnsmith".
Cancel the dialog and try to launch Tunnelblick again. macOS will present the dialog with the username filled in (in our example, it would say "John Smith"). Leave that alone and type in your password.
- If you are lucky, that password will work, and you can proceed.
- If you are unlucky, you have changed the password since creating the account on your computer. Try old passwords if you can remember them.