Why Bref cannot load bootstrap: “Error: Couldn't find valid bootstrap(s): [/var/task/bootstrap /opt/bootstrap]”?

I am investigating on how I can create an AWS lambda in php using the bref library

Therefore, according to documentation I set up the environment with the following command cocktail:

sudo -H npm install -g serverless
composer require bref/bref

Then using the following command created my first php lambda: