You must find them all before completing your playthrough. IT'LL NOT WORK if you collect any missing coin replaying chapters after beating the game or collect them on New Game+.
If that happens You'll need to start a new game and collect them all again so to avoid any frustration I recommend you get them in the order they are meant to be found as you progress in the game. Don't forget to talk to iChing every time you collect 3 coins to have your fortune read and she can give you your next task. Also when talking to her always select the option to explain your fortune. EDIT: As some players have posted on the comments you can collect any missing coins replaying chapters then talking to iChing in Rengkok to get the achievement. Maybe the devs patched it or perhaps it just glitched on me but nevertheless you can try collecting the missing coins via chapter selection first and if doesn't pop for you then you try following my original solution. One way or the other will get you the achievement.
Here's a video guide showing the location of all iChing's coins: