After creating all the necessary files run the below CLI commands:
1. php bin/magento setup:upgrade
2. php bin/magento cache:clean
3. php bin/magento indexer:reindex
If you found that the CSS and JS files are missing after running the upgrade kindly run :
4. php bin/magento setup:static-content:deploy
In certain version of Magento 2 it will throw some error if the action tag is left empty. So simply add a default controller in it e.g action=”catalog/product/” which will take you to the product grid when you click on any of the sub-menu.