Reciting al-fatiha is a rukn like the first takbeer so if one forgot to recite al-fatiha is the entire salah null like if they forget the Tajweed or just that Rakah? And what if they are in doubt and not certain that thet forgot?
They can ignore doubts as doubts do not replace certainty.
If one forgets a pillar (Fatiha) and doesn’t offer it, that Rakah is void. But if he doesn’t add another Rakah in its place, his prayer is void and must pray again. But if he remembers and offers it while in prayer, his prayer is valid. He must offer sujood as sahu at the end after Salam.