JSFiddle - React, Tailwind, and code Playground

HTML

<select id="ni_machine" name="ni_machine">
    <option selected value="default">Default</option>
    <option value="Custom">Custom</option>
</select>
<button onclick="sub();">submit</button>

CSS

.error {
    box-shadow: 0 0 5px red;
}

JavaScript

function sub() {
    var model = document.getElementById('ni_modelno').value;
    if (moodel == "default") {
        document.getElementById("ni_machine").className = "error";
    }
}