Clarifications on webinar role permissions needed

Having challenges with assigning hosts other than myself for webinars - challenges that I don't have when I have been made host on others' Zoom webinar accounts.  

I am default host for my account when scheduling the webinar. During setup I try to add an other email to assign as co-host/host and get this error:

Unable to assign ("email address") as an alternative host because the user cannot be selected at this time

Clarification #1: The email I am trying to add is not affiliated with my Zoom account, but I don't think that should matter?  

Since I couldn't assign an alternative host at setup, I logged into the live webinar and made her a host at the beginning.  After I dropped off, she was then trying to make a panelist co-host during the live webinar and it would not let her do so - even though this is something I have been able to do as host myself on another Zoom webinar account just last week.  Clarification question #2:  Why would she not be able to make someone else co-host if I have successfully assigned her hosting privileges?  Again, only just last week I was on someone else's Zoom webinar account and was made host during the webinar. I had no issue myself then making a panelist co-host during the call.  Not sure under which circumstances this would not work since that is what my colleague experienced yesterday per above.