Question: What must one read in salah if one does not know any surah?
Answer: One reads an aayah or more. One should repeat the aayah or aayahs to reach the number of letters that are in the Fatihah. If unable to recite any aayah of the Quran correctly one can make some dhikr, like "Al-Hamdulillah" to reach the number of letters. If unable to do even that, one keeps silent for the amount of time it takes to say the Fatihah.
... and Allah knows best.