Hi There,
Thanks for posting in.
Upon checking, you have the following custom CSS:
.coupon {
    font-weight: bold;
    font-family: Verdana;
    font-size: 32px;
    text-decoration: underline;
    text-decoration-style: dotted;
    cursor: pointer;
    display: none; /*This line hides the coupon field by default. The coupon field should be visible so customer can enter the value before clicking Apply Coupon button*/
}
Please remove that line from exit_style.css file.
Hope this helps.