Run the Group Policy Editor again, and configure the enable "Require additional authentication at startup" settings so they look like this:.
Also, check the "Allow Bitlocker without a compatible TPM" box:
Once this is done, you can finally configure a PIN, but since you can (and should) use characters and numbers, it should be probably be called password instead: password (mistakenly called PIN):
manage-bde -protectors -add C: -tpmandpin
Sometimes if you changed to BIOS settings, your system needs the the BitLocker Drive Encryption recovery key to boot.
Once that is done, you should suspend and then resume the BitLocker protection in the BitLocker Drive Encryption control panel.