This can happen if Steam does not complete the registration process so the account is not fully created and activated, when creating an account through Steam.
This is typically a browser issue with the Steam registration process. Please attempt the registration again through Firefox as we have had successful reports using that browser. Please make sure to click Log in here next to "To log in via Steam".
If this continues, please reach out to Steam for a refund and try to create the account directly through iRacing.com.