Problem with serial number

Hello,

I buy serial number here is what I have an error: [18-Nov-2017 13:36:15 UTC] PHP Fatal error: Call to a member function get_meta () on array in / home / lcpassfr / public_html / wp-content / plugins / serial-numbers / includes / checkout.php on line 64

In addition to its when I validate the command from the site its tells me Internal Error Server

Loic