Wearing a mask is a drag when you have a phone that unlocks with facial recognition. It’s doubly a drag when you also use your phone multiple times each day to pay for things at stores and restaurants, which is exactly when you’d be wearing a mask.
When you can’t use your face or a fingerprint, Apple requires a passcode to unlock the phone or use Apple Pay. Apple recommends a six-digit passcode to protect your login. The passcode has numbers from both sides of the number pad that comes up when you need to enter the code, thus requiring two thumbs to type it in. Unfortunately, the pressure to enter the code quickly, especially while paying at a grocery store, makes that very difficult.
There are online tutorials that try to teach us how to get an iPhone to recognize a face while wearing a mask. There are also YouTube videos showing a person covering up the right half of their nose and mouth with a mask and doing the Face ID setup, and then covering the left half while setting up the alternate Face ID.
It's not as easy as it looks, especially when your iPhone responds that your face was obstructed and it wouldn’t complete the Face ID setup even once.
Your best bet is to reset your passcode to one using numbers that be can easily typed with one thumb. You do have the option of a four- or six-digit passcode.
When you set up your passcode or want to change it, Apple will ask you to set a six-digit passcode, but you will see an option on that page to change the passcode option to set a four-digit code instead.