I am unable to add an alternate host.

Welcome to the Zoom Community, @Dave_Neville.

There are two requirements for an Alt-Host:

1. They must be on the same Owner’s account.
2.  They must be a Licensed (paid) account.

Based in this, with the Dell email address, I assume that Karl and you both work for Dell? And the fact that there was no error message indicating the potential Alt-Host was not in the same account, I’m assuming that Karl has a Basic account under the Dell organization’s Zoom account.  I’d contact your internal Zoom Admin and ask if Karl can be upgraded to be a Licensed user. 

If you don’t both work for Dell, let me know how the accounts are related, and I’ll see if I can help. 

Here’s a Zoom Support article that might be of some help:

https://support.zoom.us/hc/en-us/articles/208220166-Designating-an-alternative-host