GetTrackedAchievements returns a list of achievement ids instead of a table (well, it does this week so?)
/run for i=1, select("#", GetTrackedAchievements()) do RemoveTrackedAchievement(select(i, GetTrackedAchievements())) end
https://warcraft.wiki.gg/wiki/API_GetTrackedAchievements
or put the return list into a table for ipairs()