Vue.reactive is not a function

This error occurs when we mismatch the version of the package of Vuex with a Vue or Nuxt application.

Correct version:
for Vue.js 2.x, you need to use Vuex 3.x
and for Vue.js 3.x, you need to use Vuex 4.x

Similarly for NuxtJS
for Nuxt 2.x, you need to use Vuex 3.x
and for Vue.js 3.x, you need to use Vuex 4.x