Q: I can't find erase free space in disk utility, where is it?

Secure delete was removed from El Capitan. There are third-party apps that can but it's a bad idea on SSD drives.

I don't. I don't worry that someone may try and recover any info from my drives once they are formatted/erased once anyway. I'm not that important.

Someone might and post.

FWIW, if you are planning on selling or giving away your Mac this should be sufficient.

What to do before selling or giving away your Mac - Apple Support

If the computer shipped with Lion through Yosemite boot to internet recovery mode to use the older version of Disk Utility.  If truly worried about security just use FileVault.