Adding script elements to the DOM will cause them to load asynchronously, not in time for code that immediately follows to make use of them. You need to use a callback function for the script's onload event. Here is an example helper function that does so.

//Helper function

function loadAsync(url, callback) {
  var s = document.createElement('script');
  s.setAttribute('src', url); s.onload = callback;
  document.head.insertBefore(s, document.head.firstElementChild);
}

// Usage -- callback is inlined here, but could be a named function

loadAsync('https://www.paypal.com/sdk/js?client-id=test&currency=USD', function() {
  paypal.Buttons({

    // Set up the transaction
    createOrder: function(data, actions) {
        return actions.order.create({
            purchase_units: [{
                amount: {
                    value: '0.01'
                }
            }]
        });
    },

    // Finalize the transaction
    onApprove: function(data, actions) {
        return actions.order.capture().then(function(details) {
           //...
        });
    }

  }).render('body');
});