According to Google, that's because:
- Retrieve, as Google can not currently retrieve your URL because the server has taken too long to reply.
- Or fetching Google canceled the fetch because too many consecutive requests were sent to the server for different URLs.
I've seen this happen a few times in the past. In most cases, it is a temporary problem that is eliminated after a short time.
Note that the URL is not accessible to all by Google
Unreachable for retrieval as a Google Simulation tool.