Due to higher security on this website, the UserID/UserName is case sensitive. If your account was created with 'yourID' as the UserName then 'yourID' must be used exactly. It cannot be entered as 'Yourid' or 'yourid'.
NOTE: Apple phones and tablets like to auto-capitalize the first letter in text boxes. This does happen on the UserName field (but not on password field). Using the example above, unless I shift to lowercase on my iPad, it will start the UserName entry as 'YourID'. This will result in an 'invalid username or password message'. This the same situation if your UserName is an email address.
A simple fix is to start your UserName with a number, like '4yourid'. This defeats the Apple device issue.
A way to find your exact UserName is to do the password reset link option, using the email address you signed up with. On the screen where you enter the new password, at the top of the box, you will see your exact UserName.