Adding custom theme template to custom post type [duplicate]

File should be single-member_post.php instead of single-memberPost.php.

In single-{posttype} , {post_type} is the $post_type argument of the register_post_type() function.

Never use flush_rewrite_rules(); in init use it only on theme/plugin deactivate or activate.

Since this is a theme you can use it on after_switch_theme hook.

add_action( 'init', 'my_cpt_init' );
function register_cpt_member_post() {
    register_post_type( ... );
}

function my_rewrite_flush() {
    register_cpt_member_post();
    flush_rewrite_rules();
}
add_action( 'after_switch_theme', 'my_rewrite_flush' );