Cannot enable FileVault on macOS High Sierra
Being on MacOS Mojave 10.14.6 the following worked for me. Process was partly derived from below mentioned reddit and https://derflounder.wordpress.com/2019/02/08/unable-to-enable-filevault-on-macos-mojave/.


Tested for all user accounts on the computer in terminal the command sudo sysadminctl -secureTokenStatus USER_NAME_HERE. It returned for all accounts "Secure token is DISABLED for user". If this is different, see below.
If for all users step 1 returned "Secure token is DISABLED for user", boot into Recovery mode (reboot and hold command-R)
In Recovery mode start Terminal window (menu Utilities -> Terminal)
Execute command resetFileVaultpassword to change the passwords for all users.
Then restart back into normal mode. For me changing all passwords resulted in TouchID becoming disabled, but I could re-enable without issues.
Now back in normal mode, terminal confirmed for command from step 1 that "Secure token is ENABLED". Go to System preferences and enable FileVault.


My understanding is that if for at least one user the return in step 1. says "Secure token is ENABLED for user", this user could be used to re-enable the desired admin user by

a) promoting the TOKEN_user to admin,

b) login with the TOKEN_user,

c) change the password of all non-TOKEN_users (according to https://www.reddit.com/r/MacOS/comments/74scld/unable_to_turn_on_filevault_on_high_sierra_apfs/do1beb1/ this will make them users with a TOKEN as well), and finally

d) change promoted TOKEN_user back to normal user.