leland_learndash_set_default_course_access_setting.php

<?php
// UPDATE: I wouldn't use this anymore
// UPDATE: I would use this instead: https://developers.learndash.com/constant/learndash_default_course_price_type/

function leland_learndash_set_default_course_access_setting( $options_fields ) {

	global $pagenow;

	if ( "post-new.php" === $pagenow && "sfwd-courses" === get_post_type() ) {
		foreach ( $options_fields as &$option_field ) {

			if ( "course_price_type" === $option_field["name"] ) {

				$option_field["value"] = "free";

			}
		}
	}

	return $options_fields;
}
add_filter( 'learndash_settings_fields', 'leland_learndash_set_default_course_access_setting', 10, 1 );