Based on reports on the MacRumors forums, Twitter, and other social networks, some people are running into a specific problem - an installation that hangs.
After downloading macOS Catalina and installing from the Mac App Store, some people have seen the installation process stop right at the "Setting Up Your Mac..." screen, and even after waiting for hours, it doesn't resolve.
Luckily, there appears to be a super simple fix for this issue: Just restart. Hold down the power button on your Mac until it turns off, and then power it back on.
Based on reports from MacRumors readers, this fix will send you to the login screen or to your desktop.
After the Mac gets to the "Setting Up Your Mac..." screen, the macOS Catalina installation is largely complete. We don't recommend restarting right away, but if sits at that screen for a half hour or more, shut it down because it's not going to resolve on its own.
The macOS Catalina installation takes some time so don't restart ahead of when the setup screen pops up, but based on a multitude of reports, resetting in this way if it gets stuck appears to be a safe way of getting your Mac up and running again.