Fatal Error when adding product to basket

Hi

When a logged-in user adds a product to their basket, an error message follows stating that an error has occurred “The site is experiencing technical difficulties”.

I ran debugging and identified this fatal error from the error log:

[14-Oct-2019 10:46:24 UTC] No products in the order
[14-Oct-2019 10:46:24 UTC] PHP Fatal error: Uncaught Error: Call to undefined function GuzzleHttp\Psr7\stream_for() in /home/staustel/public_html/wp-content/plugins/woo-mailerlite/vendor/guzzlehttp/psr7/src/Request.php:53
Stack trace:
#0 /home/staustel/public_html/wp-content/plugins/woo-mailerlite/vendor/mailerlite/mailerlite-api-v2-php-sdk/src/Common/RestClient.php(96): GuzzleHttp\Psr7\Request->__construct(‘POST’, Object(GuzzleHttp\Psr7\Uri), Array, ‘{“cart_data”:{“…’)
#1 /home/staustel/public_html/wp-content/plugins/woo-mailerlite/vendor/mailerlite/mailerlite-api-v2-php-sdk/src/Common/RestClient.php(56): MailerLiteApi\Common\RestClient->send(‘POST’, ‘woocommerce/sav…’, Array)
#2 /home/staustel/public_html/wp-content/plugins/woo-mailerlite/vendor/mailerlite/mailerlite-api-v2-php-sdk/src/Api/WooCommerce.php(58): MailerLiteApi\Common\RestClient->post(‘woocommerce/sav…’, Array)
#3 /home/staustel/public_html/wp-content/plugins/woo-mailerlite/includes/shared/mailerlite-wp-functions.php(390): MailerLiteApi\Api\WooCommerce->sendCartData(‘https://www.st in /home/staustel/public_html/wp-content/plugins/woo-mailerlite/vendor/guzzlehttp/psr7/src/Request.php on line 53

There are no other errors other than this one from Mailerlite.

We rely on the automations from MailerLite and need this working asap so your help as soon as possible will be appreciated. In the meantime, I have had to deactivate the Woocommerce MailerLite plugin.

MailerLite 1.4.5
WordPress 5.2.3
Woocommerce 3.7.1
PHP 7.2
PHP Memory Limit 256M
WP Config Memory 64M

Kind regards

Jeremy