This extension is part of the TYPO3 Core, but not installed by default.
Check whether you are already using the extension with:
composer show | grep adminpanel
This should either give you no result or something similar to:
If it is not installed yet, use the
composer require command to install
composer require typo3/cms-adminpanel
The given version depends on the version of the TYPO3 Core you are using.
In an installation without Composer, the extension is already shipped but might not be activated yet. Activate it as follows:
In the backend, navigate to the Admin Tools > Extensions module.
Click the Activate icon for the Admin Panel extension.