JSFiddle - React, Tailwind, and code Playground

HTML

<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>
<button type="button">Submit</button>

CSS

.error{
    border: 1px solid red;
}

JavaScript

function checkEmptySelect() {
    return $('select').filter(function() {
        return !this.value.trim();
    });
}

$('select[id^=selected]').on('change', function() {
    var unselect = checkEmptySelect()
    if (unselect.length) {
        unselect.addClass('error')
    } else {
        $('select[id^=selected]').removeClass('error');
    }
})