@Damanya the king approves all the marriages in a kingdo and in this case the king has denied claude the right to marry camilla making it "unlawful". so currently if camilla and claude were to get married it wouldn't be recognized by the church and she wouldn't be his wife in lawful marriage and she wouldn't have the right to be called the duchess.
he specifically called it unlawful because, as stated, our FL lineage is unknown. she currently, after being outed as a lady instead of a man, is being questioned on the basis of if she is even of her fathers name and blood to begin with.