I just wanted to say I have this problem too.
This is the first time I've ever seen this in nearly 20 years of administering and supporting Macs professionally.
The user I'm trying to delete may have been brought over to the new system via Migration Assistant, but I can't be 100% sure.
I have an admin user on the Mac, that I'm logged in as.
There were two user accounts on the Mac as well, one that I was able to delete (that's only existed for a week or two) and one that I can't delete as it's asking for the user's password.
I have entered the user's password, and I know with 100% certainty this is the correct password as I can use it to log in, and the dialog box just shakes it's head at me.
The computer is a Mac Pro 2010 model (tall tower) running High Sierra 10.13.5 on a 3rd Party SSD (Intel) that is APFS. FileVault is NOT enabled.
The admin user was created programatically (createUserPkg or something like that), and has UID 499. The user I'm trying to delete has UID 502.
I've run the dscl command and it shows the user I'm deleting has a secure token ENABLED.
If I run sudo sysadminctl -deleteUser <username> it says:
User <username> can not be deleted (it's either let admin user or last secure token user neither of which can be deleted)
I don't know however how to assign my admin user a Secure Token and then remove the token from the regular user, or delete the regular user altogether...