asyncExample.js

const AWS = require('aws-sdk');

const client = new AWS.SecretsManager({});

// Call the AWS API and return a Promise
function getAwsSecret(secretName) {
  return client.getSecretValue({ SecretId: secretName }).promise();
}

// Create a async function to use the Promise
// Top level await is a proposal
async function getAwsSecretAsync (secretName) {
  var error;
  var response = await getAwsSecret(secretName).catch(err => (error = err));
  return [error, response];
}

// Call the async function and return NodeJS callback style
module.exports = function asyncExample () {
  var [error, secret] = getAwsSecretAsync('dev/MySecret/MyService');
  
  if (error) {
    // Trigger an error and halt
    console.error(error);
    return;
  }
  
  // Use the result
  console.debug(secret);
}

This code works :

const clientsecrets = new SecretsManager( { region: 'eu-west-3' } );

  const getMySecret = async ( SecretId ) =>
  {
    const s = await clientsecrets.getSecretValue( { SecretId } ).promise();
    return s.SecretString;
  };

  const secret = await getMySecret( 'STRIPE_PUBLISHABLE_TEST_KEY' );
  console.log( 'secret : ', secret );

I'm getting this error SyntaxError: await is only valid in async functions and the top-level bodies of modules

const secret = await getMySecret( 'STRIPE_PUBLISHABLE_TEST_KEY' );
console.log( 'secret : ', secret );

This block of code needs to be wrapped in an async function. Node.js doesn't, yet, natively support calling await outside of an async unlike Deno.

To fix your issue, you can do this:

// Either
// using an IIFE (Immediately-invoke Function Expression)

(async () => {
  const secret = await getMySecret( 'STRIPE_PUBLISHABLE_TEST_KEY' );
  console.log( 'secret : ', secret );
})();

// or just use a Promise expression
getMySecret
  .then((secret) => console.log( 'secret : ', secret ))
  .catch((error) => console.error(error));

Top level async/await is now available in Node.js LTS (v16.15.0 as of now).