He probably accidentally added a space at the start or end, or his keyboard is typing some special characters that were not recognized but looked like a letter.
I would have tried copy pasting SUN from this topic (or a text document) into that box rather than typing it out.
If it worked for everyone else then something must be wrong on his end.
Maybe his CapsLock is on or something
Try a different browser! Different device. Clear your cookies or what not.
The question should read “Who has the most beautiful flashlight in all the land?” Then the answer should be “sb56637” 
Supposedly the answer is case-insensitive. I did type the answer SUN in all caps, which shouldn’t make any difference, but maybe that was the source of the problem.
Signed him up on my computer and it worked. I personally tried signing him up on his computer numerous times. No locks were on, tried copy and paste, both caps and no caps were tried. His computer is new and running Windows 10, I don’t know. Thanks for all the help.
Maybe clearing his browser cache and cookies would have helped?
If anyone has this problem in the future they should try that.