Mark is having trouble encrypting his hard drive in Windows 8.1. He's told it's encrypted by default, but Leo says if he can't get it turned on, then his hardware probably doesn't support it. Mark should look for TPM 2.0 support. Users also need support for Windows connected standby feature. So if he doesn't have all that, he'll have to get a third party encryption utility. TrueCrypt is free and open source, but unfortunately, they've given the government a back door.
If Mark can find an older version of TrueCrypt, that's the way to go. Steve Gibson has a great write up on why it's still safe to use TrueCrypt.
Mark should also check out this article at ArsTechnica.com.
For turning on BitLocker, go to GPEdit.MSC to turn it on with a PIN. But remember, he'll have to save the certificate. He should back it up in case he loses it.
VeraCrypt is a good option as well. It's based on TrueCrypt.