Your 27-inch iMac includes security features to protect what’s on your computer and to prevent unauthorized software apps from loading during startup:
Secure storage: The storage drive of your 27-inch iMac is encrypted with hardware keys to provide advanced levels of security. In the event of a catastrophic failure, data recovery may not be possible, so you need to backup your system to an external source. You can set up Time Machine or another backup plan to regularly back up the contents of your computer. See Revive or restore Mac firmware in Apple Configurator 2.
Learn more. See the Apple Support article About the Apple T2 Security Chip.