No Package Id returned after executing CreatePackage

Hello, I was testing to create a package under sandbox mode. However, the package was not created, and an error was thrown instead. The exception message was: $exception {"Could not create a new package. Exception: Object reference not set to an instance of an object."} Silanis.ESL.SDK.EslException The error occurred when I executed CreatePackage(DocumentPackage documentPackage), and there was no packageId returned. Could you help me to solve this? Thank you