Upgrading from macOS High Sierra 10.13.6 to macOS Catalina 10.15 is a significant jump that transitions your system to a 64-bit-only environment. Before you begin, you must verify your hardware compatibility and prepare your data, as this update reformats your drive and drops support for older 32-bit applications. Phase 1: Compatibility & Preparation Updating and Upgrading macOS High sierra - Apple Community
If your Mac is older than 2012, you cannot install Catalina or newer versions natively. In that case, consider staying on High Sierra or using a patching tool (not recommended for security). how to update macos high sierra 10136 to 1015
Identify Incompatible Apps: Go to the Apple menu () > About This Mac > System Report > Software > Legacy Software to see which apps will no longer work. Upgrading from macOS High Sierra 10
Your Mac will restart. The installation process usually takes 30 to 60 minutes. Do not close the lid or turn off the power during this time. Troubleshooting & Alternative Paths In that case, consider staying on High Sierra