Must prayers that were performed in a garment believed to be opaque, but later turned out to be translucent, be made up? And is ignorance of covering the awrah considered an excuse?

Concealing the 'awrah (intimate parts) is a condition for the validity of prayer. The covering must prevent the perception of skin color, meaning it should not reveal the color of the body. If it is revealing, the prayer is invalid. However, if the skin color is not apparent through it except upon close scrutiny and careful observation, or by shining sunlight on it, then it is considered a covering, and prayer performed with it is valid. Nevertheless, according to the Malikis, it is disliked, and the prayer should be repeated within its time. If the skin color is only visible upon scrutiny, and you were certain that the garment was not revealing, but then it became apparent that it was, your prayer is valid, though it is better to acquire another garment.

Concealing the 'awrah is among the commanded acts, and the rule is that ignorance is not an excuse regarding commanded acts. Thus, the obligation remains, though the sin is lifted. Assuming this garment is revealing of skin color and prayer performed with it is invalid, the default is that you should repeat the prayers you performed with it. However, some scholars, such as Sheikh Al-Islam Ibn Taymiyyah, excused the ignorant person for neglecting obligations and conditions, stating that such a person is not required to make up the missed prayers.