According to Customizing the Notarization Workflow | Apple Developer Documentation, it’s available since XCode 10.
You are running an xcode9.4 image.
As checking the config at https://config.travis-ci.com/explore shows, osx_image is not inherited into jobs (presumably because during matrix expansion, os defaults to linux, causing osx_image to be ignored). Reported it at `osx_image` is not inherited into explicitly included jobs.
A workaround is to place osx_image under the OSX job entry:
matrix:
include:
- os: osx
osx_image: xcode11.3
- os: linux