Do I have to have a passcode for this phone I15

You can use it without passcode but then you also cannot use face id. For the safety of your iPhone and the data on it I strongly reccommend against using it without a passcode

If you do not use a passcode, anyone who picks up your phone will have complete access to all of the data on the phone and they will also be able to reset it and set it up again as their own, locking you out of your own phone.