Magento – no commands defined in the “setup” namespace. in Magento2

Simple, try the following commands. I have encountered this error, and I have overcome.

rm -rf var/di/* 
rm -rf pub/static/_cache/* 
rm -rf pub/static/frontend/*
rm -rf var/generation/* 
rm -rf var/cache/*
rm -rf var/page_cache/*
rm -rf var/view_preprocessed/*

In magento 2

var/di has been replaced by generated/metadata

var/generation has been replaced by generated/code

I switched to Developer Mode, and deleted the below files.

rm -rf generated/metadata/* 
rm -rf pub/static/_cache/* 
rm -rf pub/static/frontend/*
rm -rf generated/code/* 
rm -rf var/cache/*
rm -rf var/page_cache/*
rm -rf var/view_preprocessed/*

never delete pub/static/.htaccess. putting star *at the end does not delete .htaccess file in the folder.

you may also need to run setup di compile to optimize and regenerate magento2 file system.

bin/magento setup:di:compile
bin/magento indexer:reindex

If you have access to the DB of the site, take a look at the setup_module table to see if your module is showing up there. It should have the name and the current module version. If it's not showing up in that table, then your configuration files are incorrect. A module only requires two files to be registered:

app/code/{{vender_name}}/{{module_name}}/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="{{vender_name}}_{{module_name}}" setup_version="0.0.1"/>
</config>

and

app/code/{{vender_name}}/{{module_name}}/registration.php

<?php
    \Magento\Framework\Component\ComponentRegistrar::register(
        \Magento\Framework\Component\ComponentRegistrar::MODULE,
        '{{vender_name}}_{{module_name}}',
        __DIR__
    );

If your module is registered correctly, then your next step is de-bugging the module's code. I would recommend looking at the layout xml files and the controller files that should be hitting, as this is the first step in any module's life cycle.