There are three common reasons.
The first is that you are trying to log in through the wrong page. Make sure you hit the log in button and do not use the sign up form.
The Second common problem is seen below.
It means you have used an illegal or incorrect symbol or character in your email. For example you have used ^ or a % or something which is not part of an email. Check that you are using @ rather than any other symbol in the middle of your email. Putting in a space before or after your email address will also cause it to not be recognised.