What is the ruling on someone who unintentionally pressed "like" on a post containing disbelief, then retracted it, and does this entail sin, and what is its expiation?

There is no sin upon you for what you did, as it was a mistake without intention, and there is no sin in a mistake. Allah Almighty said: "And there is no blame upon you for that in which you err, but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful." (Quran 33:5). You should now increase your istighfar (seeking forgiveness) and righteous deeds, and do not preoccupy yourself with thinking about this matter that occurred as a mistake without intention. In the future, you should only express admiration for what you have read and know to be free from any Shariah prohibitions.