Short answer
HubSpot's KB: if a crawler can't index a page, it returns a crawling error. Rescan from the page's Optimization tab. For Scan blocked by robots.txt on an external page, add the HubSpot Crawler user agent as an exemption.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
The same errors can come from external tools like Semrush crawling HubSpot pages.
3. Related
SEO recommendations and robots.txt.