You can delete your account by going to https://www.hackerrank.com/settings/account. Once you are on the page you should scroll down until you see the delete button. Please be aware that all data will be permanently lost if you delete your account.
If you signed up via a social media account, (e.g. Google, Github, Facebook, etc.) you can still create a password to delete your account. To do this, I'd recommend that you first reset your password via the "Forgot your password" link on this page https://www.hackerrank.com/login.
After resetting your password, to delete your account, please go to this page https://www.hackerrank.com/settings/account and scroll down to find the delete button. To complete the deletion process, you'll have to enter your password again.