Short answer
HubSpot's KB: ALREADY_ENROLLED means another sequence is active. UNSUBSCRIBED_PERSONAL means they opted out of one-to-one email. A wrong computer clock can put the send time in the past. After a rate limit, re-enroll at the next step.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Check the computer clock when the error makes no sense.
3. Related
Workflow enrollment and sequence settings.