Is it valid to prostrate on a barrier such as a skullcap or a head covering, given the possibility of pressing on it to reach the ground?

It is best to prostrate with the forehead directly on the ground without an obstruction. However, if a worshipper prostrates on a skullcap or similar item, their prayer is valid according to the majority of scholars, who cite the actions of the Companions who used to prostrate on their turbans. Some jurists consider it permissible but disliked. Sheikh Ibn 'Uthaymeen holds that prostrating on something connected to the worshipper (such as a skullcap, ghutra, or thobe) is disliked unless there is a need. As for prostrating on a separate item like a prayer rug or a large handkerchief, there is no harm in it.