~/pset1/ $ check50 cs50/problems/2020/x/mario/less
GitHub username: william ********
GitHub password: ********
Make sure your username and/or password are valid and check50 is enabled for
your account. To enable check50, please go to https://submit.cs50.io in your
web browser and try again. If you're using GitHub two-factor authentication,
you'll need to create and use a personal access token with the "repo" scope 
instead of your password. See https://cs50.ly/github-2fa for more information!
  • Do you use two-factor authentication? If not you maybe didn't auth the cs50 app/bot? Oct 23 '20 at 22:27

It's saying that it didn't like your login credentials. Either the username, password, or both are incorrect. It could also, more likely, mean that you haven't completed the setup to allow check50 authorization to log you in.

Go to the links given and read details on what to do.

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