There are different ways to down
There are different ways to downgrade.
Flash in recovery with recovery ROM.
Flash in fastboot with fastboot ROM.
Flash in fastboot using miflash.
If you can't backup your current ROM, make sure you download the ROM you are on so you can flash it again if needed, and backup your data.
I'm not sure about the 'required key' error. Maybe your data is encrypted? Do you have a password/pin set on your phone? It is recommended to cancel the password while you do flashing and backups.
grade.
Flash in recovery with recovery ROM.
Flash in fastboot with fastboot ROM.
Flash in fastboot using miflash.
If you can't backup your current ROM, make sure you download the ROM you are on so you can flash it again if needed, and backup your data.
I'm not sure about the 'required key' error. Maybe your data is encrypted? Do you have a password/pin set on your phone? It is recommended to cancel the password while you do flashing and backups.