Fix: Force restart (hold power button). The installer might resume correctly. If not, restore from Time Machine backup and try again after running Disk Utility.
Try each app you rely on:
This is the biggest shock for High Sierra users. Catalina completely removes support for 32-bit applications. High Sierra runs them fine; Catalina will refuse to open them. how to update macos high sierra 10136 to 1015
Alternatives: Find 64-bit updates for those apps, replace them, or accept you cannot upgrade. Fix: Force restart (hold power button)
This is the most significant hurdle when upgrading from High Sierra to Catalina. macOS Catalina completely drops support for 32-bit applications. High Sierra can still run both 32-bit and 64-bit apps. To check which of your apps will break: Alternatives: Find 64-bit updates for those apps, replace
Older versions of Microsoft Office (pre-2016), Adobe Creative Suite (CS6 and earlier), and many legacy games will cease to function. You must either find 64-bit alternatives, update to newer versions, or accept that these programs will be lost.
Fix: Boot into Safe Mode (hold Shift at startup) and clear caches and Time Machine local snapshots: