The issue for several years, has been that the final three letters need to be in the proper "case"
Either UPPER or lower case. Problem is, we've gone back and forth on this so much over the years, I forget which it is !!!
Set the last three letters all to Upper case and try again....if that doesn't work set them all to lower case and try again. (Sometimes you need to blank-out the box...click in another box then, come back and re-type the whole number to get the change to "take")
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*