Hello, @Azager,
sorry for the inconvenience the code was wrong. Below is the working code you can use to solve your issue. Here is the result image for a better reference: https://postimg.cc/HV83M9wC, as you can see in the image the code works, and the header is gone.
.page-id-11 .site-header{display:none !important;}
Regards 8Themes Team.