For a step-by-step walkthrough of the entire upgrade process: How to Upgrade to Catalina Mac OS X 10.15 Gemsbok Apple Tips and Tricks YouTube• Dec 4, 2019 Troubleshooting & Tips macOS - How to Upgrade - Apple (SI)
Use this direct link to the installer package (requires a fast internet connection): https://itunes.apple.com/us/app/macos-catalina/id1466841314?mt=12 how to update macos high sierra 10.13.6 to 10.15
Your drive has a read-only snapshot that conflicts. Fix: Restart, hold Cmd + R to enter Recovery Mode. Open Disk Utility → First Aid on your main drive. Then try the installer again. If that fails, you may need to erase the drive (from backup) – last resort. For a step-by-step walkthrough of the entire upgrade
The installer is approximately 8 GB. Once downloaded, the "Install macOS Catalina" app will launch automatically. if you intend to make a bootable USB installer (Method B in Section 6). If you wish to upgrade directly, proceed with the on-screen prompts. Then try the installer again