can’t get custom category

Hi
I want to get custom category for CPT created by this plugin.
I used get_the_terms($cpt_id, $cpt_category) function and it returned WP_Error object.
Here $cpt_id is id of CPT created by this plugin and $cpt_category is custom category created by this plugin.
And also I confirmed the $cpt_category was assigned to this post correctly.

Could you please help me?