The Shopware 6 plugin is a completely separate product from the previous version and is NOT compatible with Shopware 4 or 5. You also cannot upgrade directly from the Shopware 5 plugin to the Shopware 6 plugin.
Also please be aware that there are 5 different plugins that are not compatible to one another and are aimed at certain Shopware versions:
Find installation instructions here: https://s-publicservices.de/e-solutions/tools-support/shop-plug-ins/shopware-plug-in
When updating from a plugin release prior to 3.2.0, you must uninstall the previous release before installing the new one, because of changes in the internal folder structure!
Improved error handling.