Redis throws the error 'cannot connect to a redis sentinel' when it can't establish a connection with the Sentinel instance. Sentinels are in charge of monitoring Redis master and slaves and causing automatic failover when needed. This error could be due to several reasons like network issues, incorrect configuration or different Redis and Sentinel versions.
To fix this issue, start by validating Redis and Sentinel's configuration settings to make sure they match. Check that the Sentinel configuration file has Redis' correct IP address and port number. Also, verify Redis is running on the specified host and port mentioned in Sentinel configuration.
If the configuration seems fine, then check network connectivity between Redis and Sentinel instances. Ensure there aren't any firewall restrictions blocking communication between them. Restarting both Redis and Sentinel instances might help too.
If none of the above steps work, try checking Redis and Sentinel version compatibility. Verify they're compatible and running the latest stable versions. Alternatively, try configuring a new Sentinel instance and connecting Redis to it. Finally, consult Redis documentation or support channels for further troubleshooting help.