I have enabled co-host for meetings in Zoom, however when I try to add an email address to the meeting, the system states we are unable to assign '***********' as an alternative host because the user cannot be selecta at this time. Co-host is enabled and I don't understand what is going wrong.
Co-hosts and alternative hosts are not the same thing. An Alternative host must be another licensed user on the same account. Its likely that the user you are trying to set as Alternative Host is on a separate account, so would not qualify. You would need to set them as the co-host during the live meeting.