Q: how do I erase free space on my Mac Book Pro

Why do you want to do that?

If you have an SSD for your boot drive, you do not need to erase free space. If you are using TRIM, that makes the information inaccessible. Free space is already scrambled up by random block number allocation, and that is adequate for all but Nuclear secrets.

For Nuclear secrets, the SSD drive is removed and shredded.

When you sell the Mac, you'll erase the entire drive from your Recovery Partition. Here you can still select a number of security passes.

As far as securely deleting files and free space Apple has removed that ability from the GUI in El Capitan. This article by Topher Kessler, one of our helpers here, explains how you cn accomplish that at the command line.

For Apple's recommendation on what to do before selling a Mac, read What to do before selling or giving away your Mac - Apple Support

There is no back door.

If you lose your encryption key, Encryption will keep your data safe, especially from you.