If a worshiper forgets the sitting between the two prostrations or prostrates only once, and remembers this while in the final Tashahhud or at its beginning, would the Tashahhud sitting suffice for the sitting between the two prostrations so that he may complete his prostrations and then recite the Tashahhud, or what should he do?

One must perform an additional rak'ah in place of the rak'ah in which the prostration (sujud) and the sitting between the two prostrations were omitted, because these are two pillars whose omission cannot be rectified. If someone doubts during the prayer that they have omitted a pillar, their ruling is the same as if they had not performed it. They must act on certainty and perform what they doubted they omitted, then perform the two prostrations of forgetfulness (sujud al-sahw) after the salutation (taslim). This applies if the doubt occurs during the prayer; if it occurs after the prayer, it is disregarded.