A minor fix (not tested though):
let newsletter = `\rRecevoir Newsletter: `;
if ($w("#NewsletterCheckbox").checked) {
newsletter = newsletter + "Oui";
}
let confidentielles = `\rDonnées Confidentielles: `;
if ($w("#ConfidentielleCheckbox").checked) {
confidentielles = confidentielles + "Oui";
} // <== notice where this bracket is
const body = `
\nNom: ${$w("#Nom").value}
\nPrénom: ${$w("#Prenom").value}
\nTéléphone: ${$w("#Telephone").value}
\nEmail: ${$w("#Email").value}
\nMessage: ${$w("#Message").value}
\n${intrests}
\n${newsletter}
\n${confidentielles}`;
Notice that I moved the bracket that was the last line of your code in your previous post.