[2020] Website cant find my username but it shows in Hiscores. How do I recover it?

If you can see the account stats on the OS hiscores, and those stats aren't present when you login, you're almost certainly logging into the wrong account. An account's username (used for logging in) can be different to an account's in-game display name (the name you see on the hiscores).

I'd recommend the following.

1. Head to the Can't Log In? page: https://support.runescape.com/hc/en-gb/articles/360001552065
2. Say No.
3. Say Email My Login.
4. Enter your commonly-used email address (it'll email a list of any logins you use that are linked to that email address).
5. Repeat for other email addresses you use, or have used (and you can still access).

If that doesn't work then...

1. Head to the Can't Log In? page: https://support.runescape.com/hc/en-gb/articles/360001552065
2. Say No.
3. Say No.
4. Click Maybe We Can Help.
5. Send a support ticket to the support team and explain what account you're trying to access. They'll be able to tell you the right account login.