Getting HTTP error 403 when trying to download macOS Mojave from App Store
Forcing the link to open in the Mac App Store app solved the problem for me. Just run this command in Terminal:
open -a App\ Store https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12

Since the App Store app opens the link just fine, the problem must be something with the browser.