Soundiiz is using the Emby Connect login process to handle the connection to your server.
Be sure to use these Emby Connect credentials when you are trying to connect Emby to Soundiiz. Use this link to create an Emby Connect account / try your login and password => http://app.emby.media/#!/startup/connectlogin.html
Also, if your Emby Connect username is different than the username you use to admin your server, you must go into Users settings and then select Admin User, then link the two usernames in the profile screen.
After that, you should also check that your Emby server is reachable outside of your local network. You have to allow "External Connections" in your Emby server settings. Here is a tutorial for that too: https://support.emby.media/support/solutions/articles/44001849007
Also, if you are using a Proxy / VPN (like Cloudflare...) in front of your server, be sure to update your Emby Network Setting "Secure connection mode" to "Handled By reverse proxy"