@sheila Hi. We no longer support that version of Pencil2D as it had critical issues that made Pencil2D constantly crash and deleting user work files.
As far as I understand we can’t properly support older MAC versions as easily because of how the updated technology used to build Pencil2D is no longer compatible with older versions of macOS; this is courtesy of Apple who are the providers of the key components to the technology framework Pencil2D use, and they don’t like providing their users with much backwards compatibility freedom 
That said, I’ll gladly ask developers around to see if there’s any possible way to put forth a version that can be supported in previous macOS versions.
Perhaps though it would be better that you try other software that has more resources to maintain previous macOS versions, so here’s also an updated list of alternative animation software that you can use instead of Pencil2D: pencil2d-software.tumblr.com/post/145767476221/when-youve-worked-all-day-on-your-animation
However If you still want to use an older version of Pencil2D, and you understand all of the previous issues as well as the fact that we won’t be able to provide support for most of the older version issues since they were already fixed, here’s a page that contains older versions of Pencil2D: https://bitbucket.org/chchwy/pencil2d/downloads/
Just…make sure you save multiple copies of the same file and take the precautions outlined in this guide Pencil2D Project File Corruption Prevention / Partial Recovery Guide