clientToken from braintree not getting?

Question

I am trying to get clientToken from braintree payment gatway ....

I am using it as demo and copied all three value from braintree acount.

//this is the values I copied from braintree
MERCHANT_ID= ''
MERCHANT_PUBLIC_KEY= ''
MERCHANT_PRIVATE_KEY=''

//this is controller function api

export const braintreeToken = async (req, res) => {
  try {
    gateway.clientToken.generate({}, function (err, response) {
      if (err) {
        res.status(500).send(err);
      } else {
        res.send(response);
      }
    });
  } catch (error) {
    console.log(error);
  }
};

//payment gatway 

var gateway = new braintree.BraintreeGateway({
  environment: braintree.Environment.Sandbox,
  merchantId: process.env.MERCHANT_ID,
  publicKey: process.env.MERCHANT_PUBLIC_KEY,
  privateKey: process.env.MERCHANT_PRIVATE_KEY,
});

I want to get client token from braintree but showing

{"name":"unexpectedError","type":"unexpectedError"}