Short answer
Suspend BitLocker on the volume(s) before you change partitions. If the tool still fails, fully decrypt the target volume, resize, then turn BitLocker back on.
Safe, working procedure
1) Back up the BitLocker recovery key(s):
- Control Panel > BitLocker Drive Encryption > “Back up your recovery key”
- Or: run in admin CMD: manage-bde -protectors -get C:
2) Suspend protection on every BitLocker volume you will touch:
- GUI: Control Panel > BitLocker > “Suspend protection” (confirm)
- Or: manage-bde -protectors -disable C: (repeat for D:, etc.)
Verify: manage-bde -status should show “Protection Status: Protection Off”.
3) Do the resize in one operation (tools that reboot to “PreOS mode” are fine once...