Is it permissible to say, "I don't know," to avoid helping my maternal uncle use social media, knowing that he will communicate with women?

Among the jurisprudential rules is that means have the rulings of their ends, and whatever leads to the unlawful is unlawful. If you predominantly think or are certain that your uncle will use the computer for unlawful purposes, then it is not permissible for you to teach him. You can avoid this by using dissimulation (tawriya) without outright lying, such as saying "I don't know," intending by that to deny his action.