[Vuejs]-Vue form validation

Change

<ul>
   <li v-for="validation in validation">{{ error }}</li>
</ul>

To:

<ul>
   <li v-for="(error,index) in errors" v-bind:key="index">{{ error }}</li>
</ul>

In vue, you must provide a key for every v-for looping.

And change your data to:

data() {
 return {
   form: {
    email: "",
    password: ""
   },
   validation: [],
   errors: [],
   message: '',
   canLogin: true,
 };
},

I made your errors variable an arrayList.