Short answer
HubSpot's KB: only professional Instagram accounts (Business or Creator) connect, and they must be linked to a Facebook page you manage. If it still fails, disconnect and relink Instagram to the page, verify in Instagram's Sharing to Other Apps settings, then try again.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
If the account still doesn't show, the KB says to contact HubSpot Support.
3. Related
Instagram carousels and connecting accounts.