All praise for ALLAAH alone
This question can be answered in 2 scenarios
(I) There is no salah for the one does not recite Umm al-Kitab (Surah Fatiha)
(A)the Prophet (peace and blessings of Allaah be upon him) said: “There is no prayer for the one who does not recite the Opening of the Book [i.e., al-Faatihah].”
(Narrated by al-Bukhaari, 714).
(B)It was narrated that “Aishah said:
“I heard the Messenger of Allah (ﷺ) say: “Every prayer in which the Ummul-Kitab (the Mother of the Book) is not recited is deficient.'”
Sunan Ibn Majah : Vol. 1, Book 5, Hadith 840
this is for him who has been in Islam for very long time. After the long time in being in Islam he must have been learnt what surah Fatiha is and this is also applicable on him who knows and can recite but do not do it deliberately his prayer is deficient. For him reciting surah Fatiha is fard(obligatory).
(II) But ALLAAH’s Messenger (peace on him) granted concessions for newbies of Islam who do not know & cannot recite surah Fatiha. So that these may learn until they may recite what they know of “Praises of ALLAAH” at ease.
(A)It was narrated from Abu Hurairah that a man entered the mosque and performed prayer, and the Prophet (ﷺ) was in a corner of the mosque. The man came and greeted him, and he said:
“And also upon you. Go back and repeat your prayer, for you have not prayed.” So he went back and repeated his prayer, then he came and greeted the Prophet (ﷺ). He said: “And also upon you. Go back and repeat your prayer, for you have not prayed.” On the third occasion, the man said: “Teach me, O Messenger of Allah!” He said: “When you stand up to offer the prayer, perform ablution properly, then stand to face the prayer direction and say Allahu Akbar. THEN RECITE WHATEVER YOU CAN OF QUR’AN, and then bow until you can feel at ease bowing. Then stand up until you feel at ease standing, then prostrate until you feel at ease prostrating. Then raise your head until you are sitting up straight. Do that throughout your prayer.”
Sunan Ibn Majah : Vol. 1, Book 5, Hadith 1060
(B)Rifa`ah bin Rafi` narrated:
“One day Allah’s Messenger was sitting in the Masjid” Rifa’ah said: “And we were with him. Then what appeared to be a Bedouin man entered to pray, but he performed his Salat in a very brief manner. He then got up and greeted the prophet with Salam. The Prophet said (returning the greeting): ‘And upon you. Go back and perform Salat, for indeed you have no prayed.’ So he returned to perform Salat then came and greeted the Prophet with Salam. So he (the Prophet) said (returning the greeting): ‘And upon you. Go back and perform Salat, for indeed you have not prayed.’ [He did that] two or three times, each time coming to the Prophet, greeted the Prophet with Salam and the Prophet saying: ‘And upon you. Go back and perform Salat, for indeed you have not prayed.’ – until the people got scared and became very worried that one whose prayer was so brief had not actually prayed. Then in the end the man said: ‘Then show me, and teach me, for I am a human who has suffered and is mistaken.’ So he said: ‘Alright. When you stand for Salat then perform Wudu as Allah ordered you. Then say the Tashahhud, and the Iqamah as well. IF YOU KNOW ANY QURAN THEN RECITE IT, IF NOT THEN PRAISE ALLAH, MENTION HIS GREATNESS, AND THE TAHLIL. Then bow such that you are at rest in your bowing, then stand completely, then prostrate completely, then sit such that you are at rest while sitting them stand. When you have done that, then you have completed your Salat, and if you leave out something, then you have made your Salat deficient.’ And this was easier on them than the first matter, because if some of this was deficient, It would only reduce the reward of his Salat, it would not have gone entirely. “
Jami` at-Tirmidhi 302
ALLAAH knoweth best