To sign-in and use an account you're required to first verify your account.
This is done through an e-mail sent to your account's associated e-mail address, that contains a link with actionable items to complete the account verification.
In order to protect the system the links that are generated for verification purposes are temporary and expire after a set amount of time. This is why you should immediately verify your account after signing-up.
For some reasons it might happen that you don't follow through with the verification right away, or your e-mail inbox has an issue and the email is "lost" or delivered too late, or our systems have an issue delivering it.
If you haven't received one, or you tried to use it too late, you won't be able to successfully verify your account and in these cases you should ask for a new one through the resend confirmation dialog form.
Once you go through the process you should be able to sign-in regularly with your credentials.