Covering the awrah is a condition of prayer being valid. If a person plays with his awrah uncovered when he is able to cover it, his prayer is not valid.
Reciting al-Faatihah is one of the pillars or essential parts of the prayer without which the prayer is not valid.
If a person fails to fulfil one of the conditions of prayer being valid, such as covering the awrah, or he fails to fulfil one of the pillars or essential parts of the prayer, such as reciting al-Faatihah, due to ignorance of the ruling and not intending to go against it, then there is no sin on him and he does not have to make up past prayers. It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) entered the mosque and a man came in and prayed, then he came and greeted the Prophet (blessings and peace of Allah be upon him). The Prophet (blessings and peace of Allah be upon him) returned the greeting and said: Go back and pray, for you have not prayed.â€¦
Narrated by al-Bukhaari, 793; Muslim, 397
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
He did not instruct him to repeat the prayers he had offered before that time, even though the man said: By the One Who sent you with the truth, I cannot do better than that. Rather he instructed him to repeat that particular prayer because there was still time left for it, and he was enjoined to offer the prayer during its time. With regard to prayers for which the time had already passed, he did not instruct him to repeat them even though he had omitted some of their obligatory parts, because he did not know that that was obligatory upon him. Similarly, he did not instruct Umar ibn al-Khattaab (may Allah be pleased with him) to make up prayers that he had not offered because he had been in a state of janaabah, because he [Umar] had not known that it is permissible to offer prayer with tayammum (dry ablution, in situations where water is not available). And when the woman who was suffering from istihaadah (ongoing non-menstrual vaginal bleeding) said to him: I bleed a great deal all the time. What do you think - should I stop praying and fasting? He told her to do wudoo' for each prayer, but he did not instruct her to make up the prayers that she had not offered.
End quote from Majmoo al-Fataawa, 21/430
And Allah knows best.
Reproduced from Islam QA