You have a small mistake in the function parameters.
Just replace the comma with a dot.
Should be like this:
$test_desc = get_field( ‘custom_desc’, $term_tax . ‘_’ . $term_id );
or
$test_desc = get_field( ‘custom_desc’, ‘term_’ . $term_id );
Here is the doc:
https://www.advancedcustomfields.com/resources/adding-fields-taxonomy-term/