how to get link using sharedLinkCreateRequestForFile

I am trying to create shared link but it is giving error

func publicUrl(for item: CloudItem, completion: @escaping(String?) -> Void){

        let client  = BOXContentClient.default()

        let request = client?.sharedLinkCreateRequestForFile(withID: item.id)

        request?.accessLevel = BOXSharedLinkAccessLevelOpen

        request?.expirationDate = Date(timeInterval: 3600, since: Date())

        request?.canDownload = false

        request?.canPreview = true

        request?.perform(completion: { (file, error) in

            completion(file?.sharedLink.url.absoluteString)

            print(file?.sharedLink.url.absoluteString)

            print(item.id)

            print(error)

        })

    }

getting error

Optional(Error Domain=com.box.contentsdk.errordomain Code=403 "(null)" UserInfo={com.box.contentsdk.jsonerrorresponse={

    code = forbidden;

    "context_info" =     {

        errors =         (

                        {

                message = "Invalid value '***number removed for privacy***0'. Shared link expiration can only be set on items owned by paid users.";

                name = "unshared_at";

                reason = "invalid_parameter";

            }

        );

    };

    "help_url" = "http://developers.box.com/docs/#errors";

    message = Forbidden;

    "request_id" = pynodyfqj6vht7t0;

    status = 403;

    type = error;

}})

what about free users?