The text on the screen says "Can't have Disadvantage against target," indicating you have disadvantage on your attack and thus can't sneak attack.
I would guess that it's that little purple symbol on Astarion's and your 3rd PC's profile pictures, but idk exactly what status effect that signifies. Right click either their portraits or their characters and then click "Examine" to bring up a window where you can check current status effects on them.
Alternately/in addition, click the normal "ranged attack" button and when you're hovering over an enemy to target them, the game should say what your chances to hit are and whether you have advantage or disadvantage, and maybe the source of said modifier.
