The Ubisoft Overlay (the menu that pops up when you hit Shift+F2) is often the vessel for these achievement calls. If the overlay is corrupted or conflicts with other overlays (like Steam or Discord), the UplayAchievementEarnAchievement function fails to initialize.
What are you running? (Windows 7 users sometimes need a specific legacy fix). uplayachearnachievement could not be located
Ensure that uplay_r1_loader.dll (or the specific file mentioned in the full error popup) exists. If not, the step in Step 1 is mandatory. The Ubisoft Overlay (the menu that pops up
If you want, I can: