Why can't I add Slack user to a Slack channel? Error message says the Slack user is deactivated but the same Slack user can send me DMs via Slack

I can't add a Slack user to a Slack channel. The error message says the Slack user is deactivated:

However, the same Slack user can send me direct messages via Slack, so they aren't that deactivated. What's going on?

I can't go to the Slack settings as I am not the admin.