you can only install the current major versions (v 20.x.y) and the last previous major version (v 19.2.1) using the cc desktop app.
to install previous versions, you must have a subscription. previous versions are not available with trials. previous versions are revealed in the apps tab by clicking dots adjacent the open/install button after clicking preferences>cc>tick show older apps.
What you can install is a different issue from what you can download. It's true that you can only download the current or immediately prior version from the CC app, but you can still install and activate any version. The trick these days is obtaining the installer in the first place.
Heather, the Animate 2017 installer is no longer officially available from Adobe. You'll have to "obtain" it elsewhere.