meta queries do not currently work on taxonomies in WP. There are plans to add termmeta, but that’s going to be a couple of updates out. Also, I don’t know what the developers plans are for ACF once it is implemented in WP core.
What you need to do is get all the categories and then loop through them and see which ones have the value you want.
<?php
$args = array(
'taxonomy' => 'category',
'hide_empty' => 0
);
$c = get_categories($args);
$c_keep = array();
foreach($c as $cat){
if (get_field('isprofession', 'category_'.$cat->term_id)) {
$c_keep = $cat;
}
}
foreach($c_keep as $cat){
echo $cat->name;
}
?>
