JSFiddle - React, Tailwind, and code Playground

HTML

<span class="error" style="display: none;">Error</span>
  
<form id="form">
    <table id="ProjectTable">
        <tr>
            <td>
                <select id="selected1" selectedIndex=0>
                    <option></option>
                    <option>o1</option>
                    <option>o2</option>
                    <option>o3</option>
                    <option>o4</option>
                </select>
            </td>
            <td>
                <select id="selected2" selectedIndex=0>
                    <option></option>
                    <option>10</option>
                    <option>12</option>
                    <option>13</option>
                    <option>14</option>
                </select>
            </td>
        </tr>
    </table>
    <input type="submit" valie="Submit">
</form>

JavaScript

$('#form').submit(function () {
    var valid = true;

    $('select', this).each(function (e) {
        if (!$(this).val()) valid = false;
    });

    if (!valid) {
        $('.error').show();
        return false;
    }
});