Do you have TPM? You can check with the get-tpm powershell command. From elevated command prompt
If so the TPM enters the password for you and your system is protected by your Windows password. Only if you don't have TPM you can enter a password.
Microsoft Windows [Version 10.0.10586]
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Copyright (C) 2015 Microsoft Corporation. All rights reserved.
PS C:\WINDOWS\system32> get-tpm
TpmPresent : False
TpmReady : False
ManufacturerId : 0
ManagedAuthLevel : Full
OwnerClearDisabled : True
AutoProvisioning : NotDefined
LockedOut : False
In addition you can enter a PIN and/or use a USB key. A pin is recommended and can be alphanumeric.
What is the best practice for using BitLocker on an operating system drive?
The recommended practice for BitLocker configuration on an operating system drive is to implement BitLocker on a computer with a TPM version 1.2 or 2.0 and a Trusted Computing Group (TCG)-compliant BIOS or UEFI firmware implementation, plus a PIN. By requiring a PIN that was set by the user in addition to the TPM validation, a malicious user that has physical access to the computer cannot simply start the computer.
Can PIN length and complexity be managed with Group Policy?
Yes and No. You can configure the minimum personal identification number (PIN) length by using the Configure minimum PIN length for startup Group Policy setting and allow the use of alphanumeric PINs by enabling the Allow enhanced PINs for startup Group Policy setting.
However, you cannot require PIN complexity by Group Policy.
You might like to read this thread - it has some discussion / explanations when someone had a similar question. Installed Bitlocker does not ask for password on computer start-up! - Windows 10 Forums