FileVault can't be turned on for the disk

Symptoms

diskutil list command output in terminal shows that recovery partition is not available:

It may be also possible to boot into recovery and it may have a different version than the OS itself.

Cause

Recovery partition does not exist or macOS is not able to see it.

When macOS is not able to see "Recovery HD" partition, it will not be possible to enable FileVault.

Resolution 1

Run Internet recovery to install the current  or latest allowed version of macOS on the affected Mac.

Resolution 2

Re-image Mac with the image that includes recovery partition.

Reference

How to check if your Mac has a recovery partition? - Hawkdive.com

FileVault requires Recovery HD partition in OS X - CNET

Creating a macOS Recovery Partition without reinstalling OSX or re-running your installer // DavidJB.com // Python, Technology, Web, Linux, and more, by David Beitey

How to Restore Mac when Recovery HD Partition is Missing? | Mac Data Recovery Blog