Hello,

You should add just shortcode ID of the HTML block, not the whole shortcode, like this https://prnt.sc/t0kaqv

As for the button, check the Prime color in the Theme Settings > Styles and colors also check the buttons tab, if you do not want to change the color globally, add this code to the Theme Settings > Custom CSS > Global:

body .single_add_to_cart_button{
color: white;
background-color:red;
}

Best Regards