Technically it kinda does have achievements in a way, you can view them by going to https://ubisoftconnect.com/en-US/ and then signing in and clicking on games and then clicking on "Immortals Fenyx Rising" and then clicking on challenges and then core.
That usually happens IF..
You have a spotty internet connection / lost internet connection..
Or Ubisoft severs for your region / system are having internet connection or are down.
What device are you using and what region..
You are not the 1st on here to report having an issue.
Finally you can try saving / closing game / turning OFF console / PC then turn everything back on - verify you have internet connection and then try again.
https://ubisoftconnect.com/en-US/game/immortals-fenyx-rising/PC/overview/
Is the browser version access (Of course use the one for your region)
If you have never checked the Tasks board have you also never checked the Ubisoft Connect Challenges? There you have Core Challenges and Weekly Timed Challenges that also offer all kinds of rewards.
You can check the Ubisoft Connect in the Main games menu page or on the web site.
https://ubisoftconnect.com/en-US/game/immortals-fenyx-rising/PC/challenges/
Ok while in game.. Press the + Pause button then go down to Usbisoft connect...
Then you get the app that comes up with Challenges / Rewards / Stats pannels.
You MUST have internet connection as it goes out to the internet to access this.
You can also access this through https://ubisoftconnect.com/en-US/game/immortals-fenyx-rising/PC/challenges/?challengesType=WeeklyChallengeOr whatever country you are from..
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Edit: oh there are CORE and TimeLimited challenges.
Anyone started their game yet? Especially if you're a Ubisoft+ member? I'm from Malaysia, one of the countries not eligible for the Ubisoft+ sub, but used ExpressVPN to get one. Tried changing to Sydney and NZ, still getting the "Unable To Start" message box. Not sure what to do now...