Captivate Offline

If Lilybiri has already answered your question, then you can disregard this. However, when I read your initial post I was a little confused at what was meant. If you're using Captivate 2019 and completely disconnected from the Internet or a firewall is blocking Captivate from sending/receiving data over the Internet, then you wouldn't be able to launch it or your other Adobe apps because it requires you to log in to Creative Cloud. So, I expect you're online but in a managed environment where you have limited control/permissions. If the question is whether you need to store projects in the cloud, I'd actually recommend against doing that and instead store them locally on your computer. My experience (and the experience of my peers) has been that storing projects on network shares, virtual/cloud drives, etc. can corrupt your project, which may be what you're experiencing.

I'd caution you against storing your project on your Desktop if your company uses a roaming profile, as this can also create issues. If you can, create a local folder on your computer to store your projects, and then copy them to your network or virtual/cloud drive a couple of times a day to ensure you have a backup in case you need it.

Connection is needed for several reasons: access to the CC app (needed for Adobe fonts) and to the Assets available in 11.5 are two of them. Captivate also tries to check for updates (Learning interactions) every time it is launched.

As for auto playing slides, that is not linked to being online. Check the Slide events On Enter and On Exit. They may be set to pause. You can also have pausing points in slides from the nature of the slide (quiz/score/D&D) or from inserted interactive objects.