A "Maximum User Per Device" error is caused when a second (or third) guest attempts to sign-up on a device (most likely a phone) when the designated number of guests allowed per device has already been reached.
Each brand determines how many users they will allow to use the same device.
This setting can be found in Dashboard >>> Cockpit >> Guests
Please note: Access to the cockpit is only available to Punchh admins.
Since this setting is device based, if another guest has already used the device to create an account, then this device is considered maxed out.