My cd-key which I bought way back in 2003 for windows 98/95 isn’t 26 characters long but 16. Now what?
just go to your battle.net account and type it in. then you will likely get a new one from there.
Essentially as andreasasp said, you need to claim the 16 Digit key to your Blizzard Account which will convert it to a 26 Digit tied to your account that you can use to install/play the game. We explain it a little more in this support article: https://support.blizzard.com/article/000030751
If you have any further issues let us know.