If you enabled the guest account on macOS, you probably want to have Display login window as be List of users (instead of Name and password).
That way, the guest sees the guest account is available and clicks on it, and then it logs into the guest account.
If, however, you do have Name and password, your guest will have to type guest in as the username. I thought you had to leave the password blank. Someone else in our office thought you had to type guest as the password. Turns out you can type anything you want in the password field, and it will let you in as guest, as long as the guest account is enabled and as long as you type in guest as the username.
That said, we have experienced the guest account not working at all on some machines (running 10.13.6) with the login window as Name and password. It doesn't seem to be consistent. Sometimes toggling guest off and back on will fix it. Other times, no. So, your mileage may vary.