Esse cara teve o mesmo problema:
Eu não criei uma subpasta separada como ele, mas posso confirmar que isso funcionou para mim com a imagem do win10 PE:
8) Final solution was to create a subdirectory called "BitLocker" in \Windows\System32, then copy C:\Windows\System32\manage-bde.exe and C:\Windows\System32\en-US\ into it. Then I performed the Commit to finalize and create my WinPE image.
9) Upon booting my WinPE environment, in the Command Prompt, I change to the "BitLocker" directory and run manage-bde.exe from there to unlock my BitLocker volumes. No errors.