1. Type "gpedit.msc" in search box.
2. Go to Local Computer Policy, Computer Configuration, Administrative Templates, Windows Components, BitLocker Drive Encryption
3. In right pane, there 3 folders: Operating System Drives, Fixed Data Drives, Removable Data Drives.
My following way will be applied on Operating System Drives, but you can try it on the other 2 folders.
Select Operating System Drives.
4. Double click "Require additional authentication at startup"
5. Choose "Enabled" and check "Allow BitLocker without a compatible TPM" in "Options" box.
Keep other options unchanged.
6. Use "Next Setting" to look up other items. Repeat 5. for each item of Operating System Drives.
After the above way, you should be able to enable BitLocker in Control Panel.