Understand the fields
An M3U setup normally uses a playlist URL. An Xtream-compatible form normally asks for a server address, username and password. These are alternative input formats; neither format creates a subscription by itself.
Copy carefully
Use the exact protocol, address and port you receive. Avoid trailing spaces. If a guide URL is supplied separately, put it into the EPG field rather than the playlist field. Labels vary between players, so check the device guide.
Keep the URL private
Playlist links can include credentials. Do not paste them into public forums, search engines or random playlist checkers. For support, describe the error and use the authorised contact channel instead of publishing your access details.