[Vuejs]-Vue call method inside api callback

That’s because the context inside the callback is different:

callback: function(data) {
  console.log(this); // you will see registerPayment doesnt exist here
  this.registerPayment(data);
},

Why not use the registerPayment directly? You can do so with:

callback: this.registerPayment

If you still want to call registerPayment from within the callback you can use arrow functions to access the outside context:

callback: (data) => {
  this.registerPayment(data);
}