I am trying to show a comment box under each post,
i have tried
<?php get_template_part( 'content', 'single' ); ?>
<?php comments_template( '', true ); ?>
but the comment box is still invisible
comment_form() is what displays the comments box where you enter a comment.
Have you enabled comments for the post? This must be done when editing a single post. If the comment options are not visible when editing the post, select the screen options link from the top right of the page and tick comments.
Have you unticked the box in the Settings->discussion to not allow comments (Allow people to post comments on new articles )?