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 that device has already been used for guest sign-up.
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 not available to all system users.
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.