MGB requires devices used for MGB related purposes to be encrypted. On Macs this is done by enabling FileVault in settings.
Open the System Settings app and navigate to the "Privacy and Security" pane. Scroll down to the bottom and you will see the option to turn on FileVault.
It might already be enabled. If it says "on", then your device is encrypted. If it’s not, you can enable it by clicking on “Filevault” and then clicking “turn on”.
Choose your recovery method. You can choose to use your iCloud account or to create a recovery key.
IMPORTANT: if you choose to create a recover key you must keep it somewhere safe as if you forget your Mac's login password, then the data on your laptop could become unrecoverable without it.
If you choose to use your iCloud account, make sure you know your Apple Account/ID password.
See this document from Apple for more information on FileVault.