Update commit authors fails with "has no attributable email"

I am an owner of an organization account. I’ve imported several repositories from a provider that is discontinuing service. When I’m offered the opportunity to map existing authors of am imported repository to GiHub users, I have been successful when using my own account, but not when using the account of a colleague. When I type his user name, a drop-down appears showing his account, as expected. But when I select his account, I’m shown a page displaying the message, “has no attributable email, try email address instead.”
If I use his email address instead of his user name, I get “Attribute without linking to GitHub user” instead.
His account profile shows his email as the primary email address.
What am I missing?
Thanks,
-Nick