Hello!

No, it is not an issue and is okay if there is henna on your nails. You will be allowed to give exam. The main reason that they don't allow to apply henna on hands before exam is that, on the day of test your digital finger print scan will be captured. If your finger prints could not be digitally scanned due to reasons like applying Henna or any other cosmetic colors, you will not be permitted to take test. So it is suggested to keep your fingers clean and free from Henna before the exam, if it is just on nails then it is not an issue.

Thank You