[Vuejs]-Why I get an error in Vue when using custom HTML tags?

You should register components globally or locally.

Global registration

Vue.component('my-component-name', {
  // ... options ...
})

Local registration (in another component)

import ComponentA from './ComponentA.vue'

export default {
  components: {
    ComponentA
  },
  // ...
}

More information about that https://v2.vuejs.org/v2/guide/components-registration.html