Ookla's CLI can throw some common error messages. Below is a list with an explanation of what it means.
Configuration - Could not retrieve or read configuration (ConfigurationError)
This is usually thrown when the CLI fails to reach the internet (internet down) or the specified server.
Configuration - No servers defined (NoServersException)
This usually means the defined server is no longer available. Remove it from your server list and try testing with a different server.
Server Selection - Failed to find a working test server. (NoServers)
Not 100% sure what causes this exception yet but it's likely when the CLI can't locate a local server. You should specify a list of servers to see if that addresses the issue.