In the theme, there was no error.
Quantity did not add to the cart because of Stock Quantity, which was limited. When adding to cart quantity set over the stock quantity that time it does not add to the cart. https://prnt.sc/sgfm31
And for the coupon, You may have changed code from the main theme file. I have replaced the theme file and now coupon working correctly. https://prnt.sc/sgfmlr
Clear the cache, if possible uninstall the plugin and complete the setup, once the site is ready then add the cache plugin.
Please check the website.