Soundiiz uses the Emby Connect login process to handle the connection to your server.
Be sure to use these Emby Connect credentials 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
Be sure in your Emby user settings to set the email of your Emby Connect account.
Also, suppose your Emby Connect username is different than the username you use to admin your server. In that case, you must go into Users settings, select Admin User, and then link the two usernames to 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"
If you are using a non-static IP, be sure to leave empty the "IP:PORT" field in the Soundiiz advanced Emby login settings empty, but fill the "Server Name" field with the exact server name value (case-sensitive).