How is it correct to interpret the Quran based on conjecture, even though most interpretations are built upon it, and Allah Almighty says: {Indeed, conjecture avails nothing against the truth}?

The statement mentioned in the question is not entirely correct. Some interpretations are definitively established, such as interpretation by the Qur'an, authentic Hadith, and what has been narrated from the Companions and the Successors. As for interpretation based on opinion, relying on linguistic indications and ijtihad (independent reasoning), it is built upon conjecture. However, it is a type of conjecture that is acted upon, and it is not the blameworthy "conjecture" intended in the Qur'an. The word "conjecture" (الظن) in the Qur'an is used with two meanings: doubt, and certainty/knowledge.