A NewsletterSubscription custom object exists that has a key attribute named email of type String...

In Salesforce B2C Commerce, when creating or modifying data that is to be persisted, such as creating a custom object instance, the operations need to be wrapped within a transaction. This ensures that the changes are committed to the database only when all operations within the transaction are successful, providing atomicity and consistency. If the script that creates the NewsletterSubscription custom object is not executed within a transaction, the object may not be saved to the database, which could be why the custom object is not persisted. Transactions are crucial in scenarios where data integrity and consistency are important.