That message refers to the security code at the bottom of the registration form. It’s case-sensitive and must be entered exactly as displayed. Here are some tips:
Make sure that you’re entering the code into the correct field, which is the space immediately below the code image
Ignore the faded background characters. Only enter the stronger foreground characters.
There should be no spaces between the characters.
If you’re on a mobile device, remember that some devices default to capitalizing the first letter of anything you type. You may need to manually uncapitalize the first character while entering the security code if the first character of the displayed code is not a capital letter.