mounted method giving error message - Nuxt.js, Jest

You can mock any component methods like

import { shallowMount } from "@vue/test-utils";

 describe('Product.vue', () => {
        it('btn display as "Select options" when there is validation', () => {
            const mocks = {
                $getVatiation: {
                  get: () => [] // returns an empty array, change to what you want to return
                }
            }
            const wrapper = shallowMount (Product, {mocks}) // send your mocks as an argument
            expect(wrapper.find('.product-btn').text()).toBe('Select options') 
        })
    })