Copy & paste the serial number so that you don't misread any of the characters. Some characters can be hard to distinguish. For example, there are no letter "Ohs" in a serial number, they are all the number zero.
Be careful with others such as:
Letter "B" and number "8"
S 5
Z 2
L 1
Plus be careful with the letter "Q" and zero as the tail for the "Q" can sometimes be hard to detect.
G and C can sometimes be hard to distinguish as well.
So it is best to copy & paste the serial number from "About This Mac" or the System Profiler into the box on the Apple website.
Once in a while a correctly entered serial number may not be recognized, but that seems to be very rare in my experience.