I have the same problem on BS (11.3, Intel) and get the same error messages. I tried several things to no avail:
a.
I first tried to recreate the deleted users with exactly the same name as recommended. macOS did indeed recognise the user name and asked if I would like to use the previous folder. I accepted and was able to log in as this user. I let macOS reconfigure itself properly, then logged out in orderly fashion and connected back to my own admin account. From there I deleted the unwanted user again, taking care to choose "delete user folder as an option". I suspected they were deleted the first time with the "leave folder in place option" and that would be the cause of the problem. Unfortunately, no success after this, everything remained unchanged.
b.
I then thought I would need even greater privileges to delete the folders and enabled the root user (it is complicated now and you should proceed with care). root was not able to delete the folder from the finder, so I went to terminal and rm -rf the folder.
No luck. I had quickly tried it before with a sudo command (before enabling root) and it did not work either.
I am out of ideas now.