PHP Fatal Error when enabling FedEx

I just installed this plugin on my site. As soon as I enable the “Method Enable Checkbox : Enable Fedex Global Shipping Method”, I receive a fatal PHP Error.

Any ideas?
All the plugins (including Woocommerce) have been updated to the latest versions.
Here is the error:

PHP Fatal error: Uncaught Error: Class name must be a valid object or a string in public/wp-content/plugins/flexible-shipping-fedex/vendor_prefixed/jeremy-dunn/php-fedex-api-wrapper/src/FedEx/AbstractComplexType.php:160
Stack trace:
#0 /public/wp-content/plugins/flexible-shipping-fedex/vendor_prefixed/jeremy-dunn/php-fedex-api-wrapper/src/FedEx/RateService/Request.php(32): FedExVendor\FedEx\AbstractComplexType->populateFromStdClass(Object(stdClass))
#1 /public/wp-content/plugins/flexible-shipping-fedex/vendor_prefixed/wpdesk/fedex-shipping-service/src/FedexShippingService/FedexApi/ConnectionChecker.php(108): FedExVendor\FedEx\RateService\Request->getGetRatesReply(Object(FedExVendor\FedEx\RateService\ComplexType\RateRequest))
#2 /public/wp-content/plugins/flexible-shipping-fedex/vendor_prefixed/wpdesk/fedex-shipping-service/src/FedexShippingService/FedexShippingService.php(241): FedExVendor\WPDesk" while reading response header from upstream, client:  request: 

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