Hi John,
I haven't tested yet to try and reproduce this and confirm my theory, but looking at the error message again, I may have identified a possible issue.
Are you trying to import this CSV from the "Import CSV" link on the related target series parent's view page? Rather than using the global Import CSV option available in the main menu to administrators?
I confess it's been a while since I've used the description view page import link, but my theory is that when this is used, AtoM automatically sets the target parent as the description shown on the view page, and will import the contents of the CSV as children. However, by setting the qubitParentSlug, it's now looking for a descendant under series-21 with that slug, rather searching globally for a description using that slug - hence the error.
If my guess about your workflow is correct, I'd recommend trying your import from the global Import menu instead. The qubitParentSlug column value will still find the right parent and attach the CSV records as children during import. Hopefully this will also bypass the error.
The other possible workflow - which I'm less sure about - would be to import it from the series-21 description view page, but do NOT set the qubitParentSlug value in the CSV, since the location of the import is already determining parentage.
Either way, I strongly recommend making a backup before you import again, so you can easily roll back if something goes wrong.
If my suggestion doesn't resolve the issue, then I'll try to reproduce the error locally, so we can determine if there's something else in the CSV causing the issue - or if it's particular to your installation.