Apple sometimes hides older OS versions. Use this direct App Store link: https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12
Before you begin — checklist
If you open System Preferences > Software Update on High Sierra 10.13.6, you might see "Your Mac is up to date." This is because Apple's CDN (Content Delivery Network) often deprecates upgrade paths for older OS versions. how to update macos high sierra 10.13.6 to 10.15
If you have a firmware password set: