How to get the "securely erase" function of Disk Utility on El Capitan & Sierra
Use diskutil from Terminal.  The following command

diskutil secureErase 0 /dev/diskX


Will erase your disk, identified by X, with a single pass of zeros.  You can get a list of the options available to you by issuing the command without any options:

$ diskutil secureErase

Usage:  diskutil secureErase [freespace] level MountPoint|DiskIdentifier|DeviceNode
Securely erases either a whole disk or a volume's freespace.
Level should be one of the following:
        0 - Single-pass zeros.
        1 - Single-pass random numbers.
        2 - US DoD 7-pass secure erase.
        3 - Gutmann algorithm 35-pass secure erase.
        4 - US DoE 3-pass secure erase.
Ownership of the affected disk is required.
Note: Level 2, 3, or 4 secure erases can take an extremely long time.




Secure Erase and SSDs

You don't need to do a secure erase of an SSD because a a standard erase is already more than enough to secure your data.  The reason you needed multiple passes or even the DoD 7 pass secure erase was because with traditional hard drives (HDDs) the data was stored on magnetic platters which left a residual magnetic imprint even when wiped.  This is how COTS (Commercial Off the Shelf) utilities like Disk Drill  software is able to reconstruct a drive. This is not the case with an SSD; nothing is magnetized.

In fact, it is recommended (read that to mean highly advised) that you don't do a secure erase because you would be shortening the life expectancy of your SSD due to the increased write operations.

Per Apple:


  Note: With an SSD drive, Secure Erase and Erasing Free Space are not
  available in Disk Utility. These options are not needed for an SSD
  drive because a standard erase makes it difficult to recover data from
  an SSD. For more security, consider turning on FileVault encryption
  when you start using your SSD drive.


Now, if TRIM is enabled (and it should be if your Mac came with an SSD from the factory), it will handle the task of freeing up the blocks of data no longer in use making them available for immediate writing.

Regular HDDs didn't do this.  When you deleted a file, it only deleted the pointer to the data, not the data itself.