What is the ruling on performing ablution while the nail polish is on fingers’ nails?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.

Referring to your question, an important condition to make the ablution valid is to let water reach all organs which must be included in ablution. Allah says:

O you who believe! When you intend to offer Aṣ‑Ṣalat (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to the ankles.” Al-Ma’idah: 6

Thus, the nail polish which is put on the nails of fingers or toes prevents water from reaching every part of hands and feet because the nail polish builds a plastic layer over nails which works as an isolation. That’s why all scholars agreed on this opinion. Moreover, one cannot consider the nail polish as the plaster cast because the latter is usually used for medical purposes and it is a necessity.