This is the step where most users get stuck. You cannot simply go to the main "Download" page on the SketchUp website today, as it will only serve you the newest version (currently 2024 or later).
To install SketchUp 2019, you must obtain the correct installer file. Install Sketchup 2019
If you encounter a unique error not covered here, visit the official SketchUp Community Forums (search “2019 installation”). Do not pay a “tech support” scammer – the solution is always free. This is the step where most users get stuck
Apple’s security changes make installing older software tricky. macOS 10.15 (Catalina) and later require extra steps. If you encounter a unique error not covered
If you have a slow internet connection, look for the Offline Installer for Windows which contains all necessary files in one larger package. 3. Step-by-Step Installation (Windows) Downloading SketchUp