Sometimes you can see notifications in Magento 2 panel that are displayed over and over again. This is Magento 2 admin notifications.
To stop showing these messages you need to mark them as read.
For this please navigate to Noficiations Manage page. Click on the bell icon in the top right corner in the admin panel and then press the "See All (X unread)" link.
If you have installed or updated Magento 2 extension and received the error message e.g.:
Module 'Company_Module' from 'app/code/Company/Module' has been already defined in 'vendor/company/module'.
this means that you have 2 versions of the extension:
1) in the folder app/code/Company/Module
2) in the folder vendor/company/module
In vendor folder Magento stores all components (extensions, themes, language packages) that have been installed using Composer.
As installing or updating extensions via Composer is preferred, you need to remove extension folder app/code/Company/Module (please don't forget about a backup before removal) and run installation commands.
In Magento 2 you cannot perform the reindex from the back-office. To make a reindex you need use PuTTY or another terminal application to run the CLI commands, e.g. command for reindex:
php bin/magento indexer:reindex
With FREE Magento 2 Command Line (CLI) Extension by Magefan you can run CLI commands directly from Magento 2 admin panel.
Once you install Magefan CLI extension a new menu item "System > Command Line" will be added to the admin menu.