Box Salesforce Flow: An Apex error occurred: box.Toolkit.BoxApiException: The folder was returned with a null id

When I have experienced this error I always check the following:
 

1. Make sure there is no existing folder of the same name (“Savings Blue Print”) within the parent folder ID. Duplicate folder names can sometimes cause issues similar to this.
2. Check to make sure your Service Account user that you authenticated the Box Salesforce package with has the appropriate level of access (e.g. Edit access) on the parent folder you are trying to reference. In most cases you must explicitly click the Share button on the parent or root folder and make sure the service account has access (the account should be visible under the sharing tab within Box when you select the folder)

Hope this helps resolve your issue!