How to disable Trusted Platform Module (TPM) in BitLocker:
  1. Open Group Policy Editor:

    If Group Policy Editor appears to be unavailable, follow instructions for enabling BitLocker first.

  2. Open Local Computer Policy > Computer Configuration > Administrative Templates > Windows Components > BitLocker Drive Encryption > Operating System Drives.

  3. Edit Require additional authentication at startup policy.

  4. Set the policy to Enabled and make sure Allow BitLocker without a compatible TPM is checked.
BitLocker on Windows 7/Vista does not support passwords for system drive if TPM is unavailable. If you do not like to use USB Startup key or memorizing BitLocker Recovery Key (which can be used like a password), upgrade to Windows 8 or later which supports passwords for system drive even if TPM is unavailable.