Problem with Vue & Stripe setup

I am using Laraval v5.4.35 & following the lessons for 'How to Accept Payments'.

The issue is ' ReferenceError: StripeCheckout is not defined'.

Here's my code & thanks for any help

Chris

    <button type="submit" class="btn btn-primary" @click.prevent="buy">Subscribe</button>
</form>

export default { data(){ return { stripeEmail: '', stripeToken: '' } },

    created(){
        this.stripe = StripeCheckout.configure({
            key: Gcnvars.stripeKey,
            image: "https://stripe.com/img/documentation/checkout/marketplace.png",
            locale: "auto",
            token: (token) => {
                this.stripeEmail = token.email;
                this.stripeToken = token.id;

                axios.post('/subscription/create', this.$data)
                    .then(response => alert('Complete! Thanks for you payment!'));
            }
        })
    },

    methods: {
        buy() {
            this.stripe.open ({
                name: 'Get Connected.Network',
                description: 'GCN 12mth subscription',
                zipCode: true,
                amount: 8700
            });
        }
    },
}