A few things might prevent you from adding fingerprints to your Mac. First, macOS limits the number of fingerprints you can store on the computer. A single user account can have up to three total prints, and you can only store a maximum of five fingerprints across all user accounts. Another requirement is that your user account must have a password. This might seem counterintuitive since you can use your fingerprint to authenticate. Think of the password as a backup in case the fingerprint reader fails. You’ll be required to supply your password at regular intervals (see below). Last, ensure that both your finger and the fingerprint reader are clean so that your Mac can get a proper read.