A few people in this Reddit thread suggested that the absence could be related to Ubisoft's decision to throw in with the Epic Games Store, presumably forgetting that Epic began rolling out achievement support a few months ago. Not that it should matter—Far Cry 5 has achievements on Ubisoft Connect, for instance, even though they're not offered on Epic—and the achievements in questions are supported directly through Ubisoft's launcher anyway.
Ubisoft eventually confirmed that the absence of achievements in Assassin's Creed Valhalla is intentional, and also said that they'll also be omitted from future games. "Achievements are not not available for this title on PC, and this is an intended change," a rep wrote on the Ubisoft forums.
"Instead of Achievements for new games on Ubisoft Connect ... we have expanded the Challenges, which provide XP and other rewards in Ubisoft Connect. We know that this is a big change for a lot of you, and we appreciate your understanding in the matter."
Update: Ubisoft is adding achievements to Assassin's Creed Valhalla after all
Ubisoft now says the absence of achievements was 'unintended'.
www.pcgamer.com
To say that people are pissed would be understatement. And this is on top of other issues they introduced with Ubisoft Connect like Photo Mode not reacting in Valhalla if there are server issues, corrupted saves in WD Legion and Valhalla, delay in purchase deliveries for 24h or more, old bugs like multiple subscription reservations at full amount for Ubisoft+...