That white screen means there is probably a syntax error in your code. When working on PHP code you should enable wp debugging https://codex.wordpress.org/WP_DEBUG
Also I notices that there is an error in what I posted
<?php
$categories = get_field('my_category_field');
// make sure it is an array, if only one is
// selected it may return a single value
if (!is_array($categories)) {
$categories = array($categories);
}
$tags = get_field('my_tags_field');
if (!is_array($tags)) {
$tags = array($tags);
}
$args = array(
'post_type' => 'post',
'posts_per_page' => -1,
'tax_query' => array(
'relation' => 'OR',
array(
'taxonomy' => 'category',
'terms' => $categories
),
array(
'taxonomy' => 'post_tag',
'terms' => $tags
)
)
);
$query = new WP_Query($args);
// loop reuslts here to display posts
?>
