The username is always your email address. If you are certain you are using the correct email address with no typing errors, look at the address bar to make sure you are at the correct login page (see What's my version? for the list of pages).
Each version has a specific login url so searching the web for ClientPay and then clicking the Login button will only work for one version.
Also, if you get this error message several times in a row but it does not say that your account is locked, you are probably at the wrong website. Your username will only lock for excessive wrong attempts if you are at the correct login page.
If you are sure you are at the correct page and the error message does not say that the username is locked, you can try to click the Forgot Password link. You will receive an email shortly that walks you through how to set a new password. If it does not recognize your email address or if you do not receive an email, reach out to ClientPay Support by sending an email to firstname.lastname@example.org or call us at 1-855-299-0476 so that we can troubleshoot.