The Qur’an is not a textbook or manual but it does contain everything a believer needs in terms of his or her beliefs (iiman). Nothing is missing.
Qur’an 7:52 We have given them a scripture that is fully detailed, with knowledge, guidance, and mercy for the people who believe.
Arabic: وَلَقَدْ جِئْنَاهُم بِكِتَابٍ فَصَّلْنَاهُ عَلَىٰ عِلْمٍ هُدًى وَرَحْمَةً لِّقَوْمٍ يُؤْمِنُونَ
Qur’an 6:115 The word of your Lord is complete, in truth and justice. Nothing shall abrogate His words. He is the Hearer, the Omniscient.
Arabic: وَتَمَّتْ كَلِمَتُ رَبِّكَ صِدْقًا وَعَدْلًا لَّا مُبَدِّلَ لِكَلِمَاتِهِ وَهُوَ السَّمِيعُ الْعَلِيمُ