If I omit a letter from a pillar or an obligation, does Sujūd As-Sahw (prostration of forgetfulness) become waajib (necessary) upon me?

If a letter is missing from a verbal pillar that is essential for the prayer's validity, like Al-Fatiha, the prayer becomes invalid if the person is capable of reciting it correctly. However, it does not invalidate the prayer if the person is unable to do so. As for the rest of the verbal parts of the prayer, they are considered Sunnah.