I can't use WP PopUp Pro. If i activate the plugin, i have

I can't use WP PopUp Pro.
If i activate the plugin, i have a error im my functions.php
I see a warning too, when i access the page op plugin:
'Please define pop up taxonomy by adding define("POP_UP_TAXONOMY", "custom_taxonomy_name"); in your wpconfig.php file.'
I did it but still experiencing the problem