ANSWER:

There’s no sujood as sahu for this as reciting another surah after fateha is sunnah and not mandatory. Even if you didn’t recite it intentionally, your Salah is still valid, though you have missed out on the reward.

If someone forgets to read a surah after fatiha in 4 rakha sunnah prayer, then he should do proatration of forgetfulness or the fatiha is enough for salah to be valid?