Longitudinal lines across the nail can be due to vitiligo, atopic dermatitis, psoriasis and if they are dark striations
then melanoma should be thought of. You should get it checked from a dermatologist to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps.