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We couldn't login to your server. Please ensure credentials are correct.

When deploying, you may run in to one of the following errors:

We couldn't login to your server. Please ensure credentials are correct.
Connection to server refused. Please double check that your login details are correct and that you are permitted to access this server.

The first thing to check would be that the username and password are correct.

SSH Connection

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.

FTP Connection

If you're connecting via FTPS, you may need to verify that the server is using a valid SSL certificate.

Firewall

Lastly, this could be something to do with the firewall on your server, if connections to your server are being blocked.