Is there an older version of Captivate that can be downloaded for a free 30 day trial that is compatible with Mac OS 10.6.8?

https://helpx.adobe.com/captivate/kb/captivate-downloads.html

you can download back as far a version 5

When I have migrated some older projects this has been very useful to migrate a few versions ahead at a time.

Typically, Adobe only provides free trials for the current version. However, I have had a similar need in the past when a laptop crashed, and Adobe Support told me at that time that if I signed up for a subscription, they could alter it to be for a prior version. There's also a 14-day cancellation window from when your subscription starts to get a full refund if you cancel. Maybe not the best workaround, but it could get you what you need in the short term.