Remove sidebars from posts

Do you have a link to the site you can share? I’m a WordPress expert and can take a look and see if it’s something simple for you.

https://retreat.hardkon.pl/2015/pierwsza-ucieczka-pierwszy-powrot/

it is in the CSS style.css of your theme:

#page-entry-content:before, #blog-entry-content:before {
    background-color: #ccc;
    content: "";
    height: 100%;
    position: absolute;
    right: 25%;
    top: 0;
    width: 1px;
}

to overwrite it, i.e. to remove the vertical line, add this into style.css of a child theme or via a cudtom CSS plugin:

#page-entry-content:before, #blog-entry-content:before { display: none; }

Thank you very much Michael: it works.