Try to use "Startup options" -> "List of users" (not name and password)
Then you will see the user and guest icons during startup. Press "Guest" no password needed.
You should be able to log in.
If you recently updated your Mac to a higher version of Catalina: There is a persisting bug in Catalina, not fixed by Apple since the first version of Catalina: After the update "Guest" asks for a password, but will never give access to the guest account.
Fix, which worked for me:
- Go to System Prefs, Users.: Turn off "allow guest users to access the computer"
- Restart Computer.
- Go to System Prefs, Users.: Turn on "allow guest users to access the computer" again.
- Restart Computer: Now you should be able to log in "Guest" without a password.