If we put three translations side by side we can get the gist of the Hebrew:
Let us hear the conclusion of the whole matter: Fear God, and keep his commandments: for this is the whole duty of man. (KJV)
The end of the matter; all has been heard. Fear God and keep his commandments, for this is the whole duty of man. (ESV)
When all has been heard, the conclusion of the matter is this: fear God and keep his commands, because this is for all humanity (CSB).
ESV is more woodenly literal for the first part, where KJV and CSB give a slightly more dynamic translation; everyone agrees on the second part; the third part shows the debate in the commentaries about the meaning of hakkol: does it mean "this is the whole [duty] for man/humans (adam)" or "this is for every man/all humans" (compare NASB, which goes the same way as CSB). People (Men?) can debate their preferences over man/humanity; that has more to do with English usage than the meaning of the Hebrew.
Now let's look at the LSB:
The end of the matter, all that has been heard: fear God and keep His commandments, because this is the end of the matter for all mankind.
The first part approximately follows the more literal ESV, the second agrees with everyone, and the third adds a phrase (in italics) that is not there in the Hebrew. I suspect that the reason is that they think (plausibly enough) that Qoheleth has both ideas in view: "this is the whole for humans" means "this is the whole duty of every human being" (the chief end of man, we might say). But by adding "the end of the matter" they are borrowing from the first part of the verse an idea that is not obviously there in the last part: there is no verbal connection in the Hebrew back to the "end of the matter" only to the "all" that has been heard. So I think it's an unhelpful gloss: it would have been better to have added "this is the whole of the matter for all mankind". On the other hand, most other translations (including the 1995 NASB) are distinctly more literal here than the LSB.