If you're using Woodmart Theme, Popup checkout will not be working as expected!

To fix this problem, you need to follow some few steps:

Elementor

1. If you're using Elementor as main page builder, You need to customise the layout of checkout form.
2. Search for "WooCommerce Hook" and place the widget bottom of the page.
3. Select woocommerce_checkout_before_order_review from Hook dropdown & update the layout.

WPBakery

1. If you're using WPBakery Page Builder as main page builder, You need to customise the layout of checkout form.
2. Search for "WooCommerce Hook" and place the element bottom of the page.
3. Select woocommerce_checkout_before_order_review from Hook dropdown & update the layout.

That's it!