Even under ideal conditions, things can go wrong. Here are the top issues when updating from High Sierra to Catalina.
Before starting, ensure your Mac hardware supports Catalina. Generally, the following models are compatible: (Early 2015 or newer) MacBook Air (Mid 2012 or newer) MacBook Pro (Mid 2012 or newer) Mac mini (Late 2012 or newer) iMac (Late 2012 or newer) iMac Pro (2017) Mac Pro (Late 2013 or newer) Step 2: Important Pre-Update Actions Update macOS on Mac - Apple Support how to update macos high sierra 10.13.6 to 10.15