Uncaught ReferenceError: grecaptcha is not defined

Recaptcha has a onload callback that will run once recaptcha is loaded. Place your code inside that callback function.

https://developers.google.com/recaptcha/docs/display

<script>
    function onloadCallback() {
        /* Place your recaptcha rendering code here */
    }
</script>
<script src="https://www.google.com/recaptcha/api.js?onload=onloadCallback&render=explicit"></script>