"Wittgenstein does not present his arguments in a succinct and linear fashion; instead, he describes particular uses of language, and prompts the reader to contemplate the implications of those uses. As a result, there is considerable dispute about both the nature of the argument and its implications. Indeed, it has become common to talk of private language arguments."
Yeah, sorry, I don't have a hundred hours to spend on some dude who decided to write half of an argument in ten different places. Maybe a link to plato.stanford.edu would've been better.
But the wiki entry is a far more succinct summation of the main points of the argument. The very first subsection, which you apparently didn't even try to read, in the wiki is "Significance" which summarizes the argument in all of...2 paragraphs.
That's actually one of the core parts of Wittgenstein's "Private Language Argument"[0]. The problem isn't so much that verification is "difficult", it's that certainty in the specific sense of intelligibly private is a logical impossibility. The implication is that "experiential qualia" have a specific intelligible meaning in language (or even more broadly in behaviour), which is to say that our "subjectivity" is personal but not private. Further implications tumble outward. For instance, Wittgenstein spends a lot of time discussing the "qualia" of pain. It was (and still is, obviously considering the rise of "AI" discussions lately) an enormously consequential argument in philosophy.
[0]: https://en.wikipedia.org/wiki/Private_language_argument#:~:t....