If your Epic Games PC installation of Assassin's Creed: Valhalla isn't launching at all - when you either try to launch from within the Epic Games program or via the shortcut that was created and it gives you nothing, no splash screen, just....nothing....this fix worked for me.
Go into your install directory (in my case C:\Program Files\Epic Games\AssassinsCreedValhalla) and double click ACValhalla.exe. That allowed the Ubisoft Connect Launcher to update, which is apparently necessary.
THEN I was able to get it to launch, either by using the existing desktop shortcut, the Epic Games program Launch button, or double-clicking ACValhalla.exe. Hope this helps someone. Skal!