Unable to assign same-org user as an alternative host because the user cannot be selected at this time

The users are all part of our university and we have an organisation-wide license.
When trying to add alternative host from the org, some users are accepted as alternative hosts while others are not.
Error message is always “unable to assign selected user as alternative host at this time”.
All the guidelines have been followed as some users are able to perform as alternate host.
Our internal IT team couldn’t resolve it.
Many thanks