Add 'author' to blogposts

where do you want to show the author?

in the same line as the date and number of comments?

in this case,
edit content-entry.php of your theme (ideally in a child theme), and edit this section:

<div class="entry-meta entry-meta-bottom">
			<time class="entry-date" itemprop="datePublished" datetime="<?php echo esc_attr( get_the_date( 'c' ) ); ?>"><?php echo esc_html( get_the_date() ); ?></time>
			<a href="<?php echo esc_url( get_comments_link() ); ?>" class="entry-comments-no"><?php comments_number(); ?></a>
		</div>

what function to add where depends on what exactly you want to show, like just the author name, or linked to the author page, or ..?

useful links:
https://codex.wordpress.org/Function_Reference/the_author
https://codex.wordpress.org/Function_Reference/the_author_link
https://codex.wordpress.org/Function_Reference/the_author_posts_link
https://codex.wordpress.org/Function_Reference/the_author_meta

example (to replace the previously posted section in content-entry.php):

<div class="entry-meta entry-meta-bottom">
			<time class="entry-date" itemprop="datePublished" datetime="<?php echo esc_attr( get_the_date( 'c' ) ); ?>"><?php echo esc_html( get_the_date() ); ?></time>
			 <span class="author"> <?php the_author_posts_link(); ?></span>
			<a href="<?php echo esc_url( get_comments_link() ); ?>" class="entry-comments-no"><?php comments_number(); ?></a>
		</div>

puts the author link in the middle between date and comments;
you can use .author in the CSS to style it if needed.