How do I remove site header from single posts?

Hello Fansylla,
You didn’t mention the name of the theme.It’s very hard to edit the post without looking at the code.
The goodz magazine theme doesn’t exist.

Add this to your CSS:

header {display:none;}

and if you just want to hide your logo:

.single .site-branding {display:none;}

Bear in mind though that if you do that your users will not be able to go back from there and you will make it difficult for them to navigate.

Rather make your logo smaller for post pages:

.single .site-branding {width:100px;}