Have you installed the patch? See the FoV
support page.
From the support FAQ
Problem
Cannot change difficulty because the slider is missing or invisible.
Solution
Quit the game and then remove this file:
Windows Vista and Windows 7: C:\Users\[WindowsUser]\AppData\Local\Divinity 2\Profile\settings.dv2
Windows XP: C:\Documents And Settings\[WindowsUser]\Local Settings\Application Data\Divinity 2\Profile\settings.dv2
If you cannot find the files, make hidden files visible. After deleting the file, restart the game.
To set Windows Explorer to show hidden files and folders, see the relevant link:
XP, Vista,
Win 7.
. Still stuck in normal mode like always.