[Vuejs]-Unable to use API key as env variable with Butter CMS library inside asyncData()

You are importing butter class from buttercms module in your component. But its not initialized. You can put your initialization into plugins/buttercms.js and add it into nuxt.config into plugin section.

import Butter from 'buttercms'

export default (ctx, inject) => {
  inject('butter', Butter(process.env.API_KEY))
}

And then you can reference initialzied instance via this.$butter in your components