You should create encryption on an empty Disk.
You can convert the existing unencrypted system to encrypted, but this is not trivial. (You should use live system to copy all of your old data to the new encrypted partitions...) Example:
- https://jschumacher.info/2016/11/encrypt-an-existing-linux-installation-with-luks-and-lvm/
- https://www.johannes-bauer.com/linux/luksipc/ Maybe, the data backup and restore (on the new, encrypted system) is the easiest than other solutions.