If someone is still looking to solve this problem, you can find the answer in the description of npmjs.com : https://www.npmjs.com/package/@getbrevo/brevo
Since the v2.1.1 you have to do like this :
const brevo = require('@getbrevo/brevo');
let apiInstance = new brevo.TransactionalEmailsApi();
let apiKey = apiInstance.authentications['apiKey'];
apiKey.apiKey = 'YOUR API KEY';
let sendSmtpEmail = new brevo.SendSmtpEmail();