I cannot turn on FileVault

As there are two accounts on this computer, it is suggested to Log Out of the " normal user " account and then into the " administrator user "

AFAIK - the ability to turn On FileVault can Only be done for the Administrator Account of this computer

If you can’t turn on FileVault on Mac

If you can’t turn on FileVault for a disk, some of the formatting information on the disk may need repair. Try using Disk Utility to repair the formatting. Disk Utility examines your disk’s formatting information for errors, then repairs any it finds. See Repair a storage device in Disk Utility.

Is there More than One User Account on this computer ?

refer to below link

Set up users, guests, and groups on Mac

If Yes, is the user logged into the Admin Account of this computer

1. Choose Apple menu  > System Preferences, then click Users & Groups.
2. Click the lock , then enter your account password.
3. Click Login Options in the lower-left corner.

Then have a few suggestions ;

1 - Get Support Choose a product and we’ll find you the best solution.Start now and open an Apple Support Ticket as they are Apple Employees to deal will these types of issues . 

2 - Have the computer evaluated by the Professionals who have the Special Hardware / Software and Expertise to diagnose the issue and offer possible solutions

And good luck with this computer issue

Verify User Account as True or False

Use the Terminal Command below

id -Gn username | grep -q -w admin && echo True || echo False

If the $USER  is admin then True will result.

Credit to @VikingOSX

Is Automatic LogIn enabled on this computer ?

refer to below

https://support.apple.com/en-ca/HT201476