def UNKNOWN_DATASET(project_id, dataset_id) -> str:
    """Error message for requests that access datasets.

    Parameters
    ----------
    project_id: string
        Unique project identifier.
    dataset_id: string
        Unique dataset identifier.

    Returns
    -------
    string
    """
    msg = "unknown dataset '{}' or project '{}'"
    return msg.format(dataset_id, project_id)