Magento 2.3 migration error : After migration missing most of the files from vendor/magento/ folder

This question was resolved in the Magento Slack community.

The issue was that they're using the actual Magento release's composer.json file, rather than their own composer.json that requires Magento itself.

First of you backup your website and Try this solution :-

If Successful migration try this.

To upgrade to Magento 2.3.x, you need to follow some more steps.

Here's the step-by-step guide to upgrade Magento to 2.3.x:

Enable maintenance mode

php bin/magento maintenance:enable 

Specify Magento packages

composer require magento/product-community-edition=2.3.5 --no-update 

Specify additional packages

composer require --dev phpunit/phpunit:~6.2.0 friendsofphp/php-cs-fixer:~2.10.1 lusitanian/oauth:~0.8.10 pdepend/pdepend:2.5.2 sebastian/phpcpd:~3.0.0 squizlabs/php_codesniffer:3.2.2 --no-update 

Remove unused packages

composer remove --dev sjparkinson/static-review fabpot/php-cs-fixer --no-update 

Update autoload

Open composer.json and edit the "autoload": "psr-4" section to include "Zend\\Mvc\\Controller\\": "setup/src/Zend/Mvc/Controller/":

"autoload": { "psr-4": { "Magento\\Framework\\": "lib/internal/Magento/Framework/", "Magento\\Setup\\": "setup/src/Magento/Setup/", "Magento\\": "app/code/Magento/", "Zend\\Mvc\\Controller\\": "setup/src/Zend/Mvc/Controller/" }, ... } 

Apply updates

composer update 

Clean the Magento cache

php bin/magento cache:clean 

Manually clear caches and generated content

Clear the var and generated subdirectories:

rm -rf <Magento install dir>/var/cache/* rm -rf <Magento install dir>/var/page_cache/* rm -rf <Magento install dir>/generated/code/* 

Update the database schema and data

php bin/magento setup:upgrade 

Disable maintenance mode

php bin/magento maintenance:disable 

Finally, check your Magento version

php bin/magento --version 

And Read More :-

https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html

THANKS.

I have an alternative solution that I have used and it did resolve my issues at the time.

After following Magento recommendations like most of us do, I still had troubles. I did install a fresh install of Magento 2.3.x. Then, once the latter was running fine,

• first thing first, I enabled developer mode: php bin/magento deplo:mode:set developer
• I did modify composer and add in this working composer file the modules that were used in my site. Then composer install needs to run to deploy these modules. At this point, you may have to change the version to comply to the Magento 2.3.x version.

--> if a module version is not working with 2.3.x, you may run composer require <module-name>. Once this step is complete, you are over the worse.

• I did deploy my modules from app/code folder into this new fresh installed site.
• I did deploy the theme file from app/design

then, run php bin/magento set:upg. At this point, you may have some issues with your app/code modules if your php version has changed. These issues usually are functions signatures to adjust to match the vendor core modules new code. Although it can be daunting, it is usually straightforward at this point