This warning category is spelled
[syntax.id-quotation]
by qmllint.
You surrounded an id with quotation marks.
This might lead to confusion between ids and strings. The QML language does not require quotation marks for ids and forbids more complex string expressions for ids.
import QtQuick Item { id: "root" }
To fix this warning, remove the quotation marks:
import QtQuick Item { id: root }