On Ubuntu Xenial the installer would stop, show insecure warning and wait for user's input to continue:
WARNING!
python3.6 package is required as a dependency for StackStorm and none of your software repositories provide it.
We recommend switching to Ubuntu 18.04 LTS (Bionic) as a base OS.
Alternatively we'll try to add 3rd party python3.6 APT repository: https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa.
By continue you're aware of security risk associated with using unofficial 3rd party PPA repository.
Per previous discussion in https://github.com/StackStorm/st2-packages/pull/679, adding an option to install 3rd party PPA based on user's input.
On Ubuntu Xenial the installer would stop, show insecure warning and wait for user's input to continue: