CloudFormation init requires the presence of cfn-init helper script on the instance. Ubuntu does not come with it, so you have to set it up yourself.

Here's the AWS guide that contains links to the installation scripts for Ubuntu 16.04/18.04/20.04. You need to add these to the user_data prop of your instance. Then cloudformation-init will work.

If you just want to create a file when the instance starts, though, you don't have to use cfn-init at all - you could just supply the command that creates your file to the user_data prop directly:

self.ec2_instance.user_data.add_commands("echo welcome > /home/ubuntu/textfile.txt")

Likely there is something else called cdk ahead of the nodejs aws-cdk package in your path. You can use the which command to figure out what path is actually being called when you run cdk. On my system, the nodejs aws-cdk package gets installed to /usr/local/bin/cdk.

Try running which cdk and if you find that your shell tells you it's running a different cdk binary, uninstall whatever that package is and retry.

TLDR; .env/bin/pip3.8 install -r requirements.txt solved this same issue at my end

Once you run source .env/bin/activate it actually looks for python libraries on .env/lib/python3.8/site-packages (at least for me!). Thus running cdk ls which calls app.py will looks for aws_cdk in venv site package, not from system site-package as long as "include-system-site-packages = false" under .env/pyvenv.cfg