Widget not visible

The modules does not provide a widget but selection plugin. You can select in field settings.

For anyone else trying to figure this out:

Go to Structure > Content type and select the content type that includes an entity reference to the taxonomy.

On the "Manage fields" page, edit the entity reference field. In the "Reference type" section, change the "Reference method" from "Default" to "Taxonomy term selection (with groups)." Change the "List item prefix" if needed and save the new settings.