Message
We could not connect to your Solr server. It’s probably because the port is blocked. Please try another port, for instance 443, or contact your hosting provider/network administrator to unblock your port.
Solr HTTP error: HTTP request failed, Failed to connect to localhost port 8983: Connection refused
When
During index configuration in wpsolr admin.
Explanation
It probably means that your WordPress hosting provider is blocking all outbound ports, but 80 and 443.
Solution
Use port 80 if your Solr scheme is Http://
Use port 443 if your Solr scheme is Https://