Thanks for the screenshot!

Just copy this on line 191:

$shortcode_2 = do_shortcode('[nostr_zap_button npub="@npub1gwa27..." url="' . $current_url . '"]');

i removed the [ and ] surrounding your npub. That's not needed.

I ll update the documentation and mention to remove the square brackets [ and ], when replacing npub. So it's easy for non-devs too.