Secure Erase with diskutil crashes consistently with OS High Sierra 10.13.6

Does your iMac use a Fusion Drive?

If you are only erasing the Free space, then it seems you are trying to preserve the OS? Why? Just create a bootable macOS USB installer so you can perform the secure erase of the Hard Drive (do not secure erase the SSD if a Fusion Drive since a simple erase of an SSD is enough to destroy all data with TRIM enabled).

If you are already booted to the macOS installer (Internet Recovery Mode or USB), then it may indicate a failing hard drive.

Edit: If you are selling this iMac, then here is an Apple article with instructions for properly preparing the Mac for sale:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

"Secure Erase" doesn't actually accomplish anything useful. You know that, right?

From High Sierra's very own manpages,

      NOTE: This kind of secure erase is no longer considered safe
        because modern devices have wear-leveling, block-sparing, and
        possibly-persistent cache hardware. The modern solution for
        quickly and securely erasing your data is strong encryption,
        with which mere destruction of the key more or less instantly
        renders your data irretrievable in practical terms.

The reason is right in front of you. The volume has insufficient free space. I understand it's 407 GB, but 407 GB is not enough.

Back up the source volume with Time Machine, erase it, restore the backup, and use FileVault.

What drive & OS are you running from?

In Disk Utility have you tried unmounting the internal drive?

Have you tried in Safe Made,orare you using Restore Made?

Have you tried Disk Utility in Restore Mode?

I suspect the disk has suffered some degree of corruption that is preventing even using DFA. I'd try simply erasing it in the usual manner (the Erase button in your first screenshot). You can always attempt a "secure erase" later.

Are you trying it using Restore Made?

Thanks, I missed that!:(