Fatal error: Uncaught Error: Interface "Psr\Http\Client\ClientInterface" not found in /var/www/html1/wp-content/plugins/zettle-pos-integration/vendor/inpsyde/psr-18-wp-http-client/src/Client.php:28 Stack trace: #0 {main} thrown in /var/www/html1/wp-content/plugins/zettle-pos-integration/vendor/inpsyde/psr-18-wp-http-client/src/Client.php on line 28