Many people on the forums claim that this game has bugged achievements. I was lucky enough not to run into these problems. But if your achievements still haven't unlocked then this save file should help you. I've tested it on two different accounts and it has worked each time.

CLICK ME[drive.google.com]

Save location:
C:\Users\(you)\Saved Games\MachineGames\Wolfenstein The New Order\base\savegame

Backup your own save before using this save

The last letter is near the exit of the asylum. Shoot the lock through the hole in the stairs and then go around into the secret room to get it.
And voila, achievement unlocked.

Like the other collectible, the last gold piece is in the aylum. After reaching the exit, the gold piece would be just under the front of the car. Get it and the achievement should unlock.
Unlock that achievement boiiisss

If this guide has helped you in any way consider sharing it so other people like yourself may benefit :^)

Other guides by me:

Show Off, From Beyond the Stars, A Mystery Solved, Close Shave SAVE FILE
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=884954525

Personal Decorator (& others) SAVE FILE https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=844159446

All Missable Achievements and their Save Files https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=707288823

Waste Management Achievement Fix https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=658564227

Peyote Plant Locations Guide https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=512104922

Giant Accomplishment https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=601209830

All Collectibles/Gear/Unlocks https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=615434440

All Collectibles/Upgrades/Activities https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=618975619

All Collectibles/Upgrades/Activities https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=619567241

Auto Fanatic/Hollywoodland/City of The Angels https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=620962071