Python command not found while installing debian 9 from iso

I have the following problem: I'm running debian 9 installation from a custom created iso with additional packages. One of the packages runs in the postinst script a python command defined in this packages, but the postinst script fails with error:

my-command command not found

therefore all software installation step fails. **my-package.postinst**:

#!/bin/bash
my-command

When postinst is commented out all the process finishes successfully. Even when the process failed ( dpkg -l return my_package in status iF) after a reboot, it is possible to run my-command from command line.
I guess the problem is in python packages which are not properly installed during running my-package.postinst, but even moving them to pre-depend did not help.
Any ideas or directions? Any help will be appreciated.
Thank you in advance, Veronika The custom Debian 9 iso disc is created using simple-cdd. pkg-root/DEBIAN/control of my-package:

Package: my-package
Version: 1.0
Priority: optional
Architecture: all
Depends: geoip-bin, openssh-server, ntp, sshguard, lsb-release, vim, less, nload, iotop, logrotate, passwd, python-requests
Pre-depends: python, apt-transport-https, iproute2, python-setuptools

my-package/setup.py

from setuptools import setup

setup(
    name='my-package',
    version='1.0',
    namespace_packages=['MyModules', 'MyModules.System'],
    packages=['MyModules.System.Tools'],
    entry_points={
        "console_scripts": [
            "my-command = MyModules.System.Tools:cli_my_command"
        ]
    },

    license='MyLicense'
)

during packaging the following command is run to create relevant entry_points,dependency_links, namespace_packages:

python setup.py install -f --install-layout=deb --prefix=/usr/local --root=pkg-root