When deploying, you may run in to the following error:
We couldn't login to your server. Please ensure credentials are correct.
The first thing to check would be that the username and password are correct.
If connecting via SSH, you'll need to make sure you've added the correct key in to the correct place, and that the appropriate permissions are set. You can do so by following this guide.
If you're connecting via FTPS, you may need to verify that the server is using a valid SSL certificate.
Lastly, this could be something to do with the firewall on your server, if connections to your server are being blocked.