Answers in this forum are given almost always by other Captivate users, not by the Captivate team. My apologies, but I am also just a user.
You may have missed the announcement that perpetual licenses are no longer available for Captivate since 15th of February. The version 11.8 is the first release under that new protocol, only available as subscription license. A subscription license is not activated using a serial key, but is linked to an Adobe ID.
If you have a perpetual license you should be able to log into your account at Adobe.com, Click on your avatar and select view account. Click on Plans and Payments and then select Products. From there you should find your perpetual license and a link to download the original software