My parents say that I won't be able to fast during to fast during Ramadan
There are five conditions for being obligated to fast:
- The person is a Muslim
- The person has reached puberty and are accountable of their actions
- The person is able to fast
- The person is not traveling
- The person has no restrictions to fasting
If all of these conditons matches yours then it will be obligatory to fast.
If you are ill, then it is okay to not fast. What defines being ill is physically not being able to fast for a short time or a long time. Your leucodema is a skin condition and it shouldn't not allow you to fast. If because of your skin condition you get really dehydrated than you can not fast.
However, if one doesn't not fast, being ill or not, one still has to pay for it. You should feed one poor person everyday of Ramadan.
Allah says in the Qur'an:
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a Miskeen (poor person) (for every day) and whoever is ill or on a journey, the same number (of days which one did not observe Saum (fasts) must be made up) from other days”
If you are able to fast and have no valid excuse to not fast , not fasting is a major sin. One must repent. Your parents cannot force you to not fast as it is a major sin for them and you.