The storage volume can be encrypted during setup. If you have already created the volume then you will need to backup your data and recreate the volume again from Control Panel > Volume.
Note: Once you had encrypted the volume, you will need to enter the encryption password or upload the key file every time when Amber is booting up. To remove encryption, you will also need to delete the volume and recreate it again.
After you have deleted the volume, click Create.
Check Encrypt this volume and enter a password.
After the volume is created, restart Amber.
After you login again, Amber will ask you for the volume decryption password.