If you did mean to use the override settings, by ticking the box as suggested by Runxel, you will also need to change the Cut Fill to Background or some other pattern that displays the Cut Fill Background Pen.
Apple iMac Intel i9 / macOS Sonoma / AC27UKI (most recent builds.. if they work)
I checked the box of the override attributes and changed the cut fill to "Background" and the Cut fill Background Pen to "Transparent" but the 2D representation of the window doesn't change. It's still white and opaque.
Try selecting the Graphic Overrides and turning them off and also check the appearance isn't being affected by the renovation view setting. If they aren't helping then it may be a bug in the library part which may even be exclusive to your localised version of AC. If you share your regional version and the name of the library part then maybe someone can confirm the problem.
Apple iMac Intel i9 / macOS Sonoma / AC27UKI (most recent builds.. if they work)
That section of the dialog box is for overriding the pens and fills in Section. Look into the window specific settings (probably in the Basic Window section) to control the 2D (plan) settings.
Can you show an image of what you are trying to make transparent?
Barry.
One of the forum moderators. Versions 6.5 to 27 i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10 Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11