Adding alternative host issue

Hi, I am trying to add someone as an alternative host for my meeting but I keep getting the error message 'Unable to assign (email address) as an alternative host because the user cannot be selected at this time'

Why might this be and how can I resolve this?

In case it makes a difference, the email I am trying to add is an external email address (not within my company)

I have checked and the email address is correct and the email is connected to their Zoom account.