that isn’t a long thin line, its a thumbnail of the site-content container ie. the container that has all of your post content inside.
And if you mean a CLS issue, then that site-content is being shifted down the page by the Advert you have loading between your site header and your site content. You need to make sure your adverts are inserted with the correct size container or you will get CLS.
Actually, that advert between header and content is showing automatically. I did not add any code for that space. Please help me with this. This long line is creating a large CLS
If it is appearing automatically then you must have some auto advert service running ?
If thats the case then i would recommend you exclude that area from auto adverts.