Achievment issues with Ubisoft games.

I also had issues with that same achievement in far cry 5 and similiar issues with other games and there are two options i normally have to do one atleast and it will work but first option is you could try unplugging your consoles for about oh 5 minutes
let the system and power cable cooldown and while they do that reset the wifi router as well just to be safe since wifi router lagging can cause achievement problems as well after that plug it in and try it again and if that doesnt work try uninstalling the
game and reinstall it after that if it still wont pop off id say send xbox an error report and see what they might say

It's sometimes stuck for a few days but they will start popping up.

some achievements takes time to unlock.. its bit glitchy.. but they'll pop..

Have you got the ubisoft club app installed? That tracks ubisoft achievements too.

Sometimes the achievements don't unlock at the first time or the specifications to unlocked are not accurate.

Try check a guide to unlocked or verify if the achievement don't have any issues to unlock it.