doesnt work for post

yeah you can fix that by following this post

it tells you to just open up replace-featured-image-with-video/pt-page-featured-video.php

find this line

function add_fv_box_fields() {
    add_meta_box( 'docs_list', __( 'Featured Video', 'related-video' ), 'admin_fv_box_html', 'page', 'side' );
}

and replace with

function add_fv_box_fields() {
    add_meta_box( 'docs_list', __( 'Featured Video', 'related-video' ), 'admin_fv_box_html', 'page', 'side' );
    add_meta_box( 'docs_list', __( 'Featured Video', 'related-video' ), 'admin_fv_box_html', 'post', 'side' );
}

then find the single.php file and look for something like this
the_post_thumbnail(‘main’)

and replace it with

if(get_post_meta(get_the_ID(), "_related-video", true) != ''){
 echo get_post_meta(get_the_ID(), "_related-video", true);
}else{
 the_post_thumbnail( 'main' );
}

my single.php file doesn’t have a line like:

the_post_thumbnail(‘main’)

Is there another file I can find it in?

same as MitchGusdal

Any help would be appreciated. Thanks!

Perhaps providing your theme name and url can be of help.