Enable BitLocker on Windows Server 2008 R2 without TPM

Enabled BitLocker Drive Encryption without TPM via gpedit.msc:
Local Computer Policy -> Computer Configuration -> Administrative Templates -> Windows Components -> BitLocker Drive Encryption -> Operating System Drives -> Require additional authentication at startup -> Select the Enabled option and then set the "Allow BitLocker without compatible TPM check box"

Source: Configuring BitLocker Drive Encryption on Windows Server 2008 R2