Why is there no operating manual on how to live life?

Everyone's manual is built upon their lived experience, and only parts of each manual would apply to others. You are building your manual through your experience to become the best version of you one day at a time. Take that which is value from which you observe and are taught by others, discard that which has no value or does not apply in your context.

"We win or we learn." Try to win more than you learn over time.

Here are a few well known ones off the top of my head:

https://en.wikipedia.org/wiki/Bible

https://en.wikipedia.org/wiki/Tao_Te_Ching

https://en.wikipedia.org/wiki/Quran

No manual because you are supposed to live your life any which way you want, but there will be consequences.

There is no one way to do that.

We don’t know much about human minds. In additional, we don’t follow manuals. The brain doesn’t really always listen to our will. TBH I’d very much hope that I’m a machine that can devote to a single thing for an undetermined amount of time, never have burnout and can immediately context switch with minimum effort. But I’m not made of that.

Why? Because we evolved, we weren't manufactured. WRT how to become the "best" version of ourselves, that depends entirely on how you define "best", and there's no universal agreement on that. That's why so many different "manuals" are available.

There is. You get to write it yourself; you get to be your own master/leader.

Critical thinking skills are the most essential tools, so is studying philosophy. You have agency, use it.

Of course you could always abdicate your agency to religion (pick one), or politicians, or influencers, or money/consumerism.

No matter how you dice it, the choice is always yours, and yours only,

… ok, lemme see here.

    mySelf = Human.new()
    defer mySelf.recycle
    while myself.not_yet_dead():
        mySelf.improve()

[delayed]

Mine is called the Bible. Which is a greek word for "the books" but commonly understood to be "the holy scriptures'. I'm happy to include Quran and Book of Mormon and all gnostic gospels into this mix; it's not important for this discussion.

Pali canon are also a personal favourite. Tao te ching is similarly good.

I havent read the bhagavad gita or the upanishads, but I should do so.

The reality here is that there's a single uncaused God. Even the polytheistic Hindus believe there's only 1 uncaused God(Brahman). But this God is too complex for human understanding. The religions are all bad, very insufficient, explanations of the God with a cultural lens to help in that understanding.

Your importance of your operating manual is to get you to the point that you live at peace with your neighbours and even your enemies. Life is too short to have enemies. Which is then complex about how to avoid ending up with enemies because cultural differences can be the break. Which is the importance of multiple religions.

Plenty of philosophers wrote those. Maybe not in manual form per se.

there’s a bunch of them, but we haven’t agreed on which one is best, yet.