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It is a self-hosted site.
I turned off multi-site and tried to the plug-in again, but got the 500 Server Error again.

I will try it on one of my other sites next. I am using the version on the WordPress Repository.

Ahh, I see. Thanks for reporting that. I'll be reviewing the WordPress MU environment for this plugin.

I was able to use the GitHub version.
It doesn't seem like scheduling the post didn't work. The post is posting automatically.