Sweet Hierarchy

Installation itself in the Module Loader works, however upon attempting to validate the license key as provided on the order page, validation fails and a dialog box comes up saying "Error: null". The loaded module for Sweet Hierarchy then disappears from Module Loader as if it were never installed. Sugar application logs provide an empty stack trace and an "emergency module uninstall" , however Apache error logs provide the following: **PHP Fatal error: Uncaught Error: Class 'GuzzleHttp\\Psr7\\Utils' not found in /var/www/html/vendor/guzzlehttp/psr7/src/MessageTrait.php:121\nStack trace:\n#0 /var/www/html/src/Security/HttpClient/ExternalResourceClient.php(310): GuzzleHttp\\Psr7\\Response->getBody()\n#1 /var/www/html/src/Security/HttpClient/ExternalResourceClient.php(272): Sugarcrm\\Sugarcrm\\Security\\HttpClient\\ExternalResourceClient->createResponse()\n#2 /var/www/html/src/Security/HttpClient/ExternalResourceClient.php(109): Sugarcrm\\Sugarcrm\\Security\\HttpClient\\ExternalResourceClient->request()\n#3 /var/www/html/modules/SweetHierarchy/license/SweetHierarchyOutfittersLicense.php(550): Sugarcrm\\Sugarcrm\\Security\\HttpClient\\ExternalResourceClient->get()\n#4 /var/www/html/modules/SweetHierarchy/license/SweetHierarchyOutfittersLicense.php(233): SugarOutfitters_API::call()\n#5 /var/www/html/modules/SweetHierarchy/license/SweetHierarchyOutfittersLicense.php(185): SweetHierarchyOutfittersLicense::doValidate()\n#6 /var/www/html/modules/SweetHierarchy/outfitterscontroller.php(16): in /var/www/html/vendor/guzzlehttp/psr7/src/MessageTrait.php on line 121, referer: https://{my-website}/index.php?module=SweetHierarchy&action=license&bwcFrame=1 ** I double-checked all Sugar directory file permissions and ownerships, as well as Quick Repair and Rebuild, cleared Sugar's cache folder, and cleared browser cache, but validation still fails. My SugarCRM version is 12.0.2.