JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" action="inserting.php" id="form1" onsubmit="return validateForm()" class="signupform" name="signupform">
    <input type="hidden" name="allowlang" id="allowlang" value="no" />
    <table cellpadding="4" cellspacing="0" width="100%" border="0" style="text-align:left;">
        <tr>
            <td>
                <input type="checkbox" name="enable" id="enable" value="YES" style="width:15px !important" />&nbsp; There has been an issue.</td>
        </tr>
    </table>
    <br />
    <table cellpadding="4" cellspacing="0" width="100%" border="0" style="text-align:left;">
        <tr>
            <td height="50"></td>
            <td>
                <br />
                <button id="registerButton" type="submit" style="float:left;">Send Data</button>
            </td>
        </tr>
    </table>
</form>

JavaScript

function validateForm() {
    if (document.forms["signupform"]["enable"].checked)
        alert("yes, there has been an issue");
}