How to deactivate captivate 2019 from an old computer

If you had Captivate installed and licensed (under subscription) on your computer then you should also have an Adobe ID that is registered on the Adobe website.  Your username is usually just your email address (because it has to be unique).

Just use another computer to log into your Adobe profile using your email address and whatever password you used when setting up the profile.

When you have logged in, look for the link to Manage Account. 

Then look for the link on the left to Activated Devices.

This should show you the names of all computers where you had Captivate installed. There should be no more than two computers listed.  When you find the computer that has now stopped working, click the Deactivate button.

You should now be able to install Captivate on another computer under your current licensing.

That is indeed a problem, if you are still on a perpetual license (no longer possible). I think you'll need to contact Adobe directly. If you have a subscription license, I don't think you still have to deactivate before installing on a new system.