Can't erase Late 2015 iMac. Error = 654 (kextcache)…Couldn’t unmount disk. :(-69888)

Unfortunately it's Step 6 where the problem starts. I hit erase as indicated in Step 6, and I get the error message "Error =654 (kextcache ...Couldn't unmount disk. :(-69888)". From there no other options to go anywhere. None of the other options on that page leads anywhere, as can't reinstall O/S, can't restore from time machine, etc. I'm hoping someone might recognize the wording of the error message! Thanks, Tom

Your guidance is MUCH appreciated. Two key comments. In "recovery mode" I have only very limited options under the Apple menu. No system preferences. However, I did find that I was able to "restore from time machine". And while it feels a bit quirky still, I do have access now to system preferences. There is no "Accounts", but Users and Groups lists what I believe you may be referring to. There is just on person listed, my name, which is listed as "Admin". I can see where I could add additional users (accounts?) could be added by unlocking, and clicking on plus sign, and adding a new admin user. That would seem to get me to the point I believe you were referencing. If I then go to disk utility, highlight the original user, and Secure Erase Free Space, what would be the next step or steps to correctly erasing the hard drive? Would I do this from the original user account, or the newly added user account? Thanks for any further guidance you're able to provide. Tom