Next.js: document is not defined

I think, in server rendering mode, the document is undefined.
You should be able to use it inside class lifecycle methods or useEffect

import React, {useEffect} from "react";
import {Elements, StripeProvider} from 'react-stripe-elements';
import CheckoutForm from '../../components/Payment/CheckoutForm';
import { useRouter } from 'next/router';


var stripe_load = () => {
    var aScript = document.createElement('script');
    aScript.type="text/javascript";
    aScript.src = " https://js.stripe.com/v3/";

    document.head.appendChild(aScript);
    aScript.onload = () => {

    };
};

function Payment({host}) {
    const key = host.includes('localhost') ? 'test' : 't';

    useEffect(() => {
      var aScript = document.createElement('script');
       aScript.type="text/javascript";
       aScript.src = " https://js.stripe.com/v3/";

       document.head.appendChild(aScript);
       aScript.onload = () => {

       };
    }, [])
    //stripe_load();

    const router = useRouter();

    return (
        <div className="Payment Main">
            <StripeProvider apiKey={key}>
                <Elements>
                    <CheckoutForm planid={router.query.id}/>
                </Elements>
            </StripeProvider>
            <br/>
            <br/>
            <p>Powered by Stripe</p>
        </div>
    );
};


Payment.getInitialProps = async ctx => {
    return { host: ctx.req.headers.host }
};

export default Payment