If you get "Error: Fetch data failed (456). Url, username or password is not correct." then it means that either you provided wrong credentials, or your IPTV provider is blocking access from outside requests except those from your IP addresses and/or your country and/or your ISP.
Thumb of rule: The website always tells the truth.
Try to import an M3U URL instead of an Xtream API URL. If that doesn't work, upload an M3U file to the site as a direct upload and link back to your IPTV provider under the "Action" button next to your M3U file. If that doesn't work either, change the IPTV provider. There are literally thousand of others out there who will provide real access.