If you are using Stripe Payments Plugin, it is overwriting the theme styling on the single product page and it breaks the layout. If you get a layout that looks like this:
Then you might need an easy fix. Please go to Appearance – Customize – Additional CSS and add the following CSS code:
.woocommerce .quantity .qty {
width: calc(100% - 80px) !important;
}
.woocommerce div.product form.cart .button {
width: 100% !important;
}