Whatsoever thy hand findeth to do, do it with thy might; for there is no work,
nor device, nor knowledge, nor wisdom, in the grave, whither thou goest.
However, any reader of Old and New Testament should know that both "books" are in fact the collections of the hand-made copies of the texts written by different authors in different times. The idea of everlasting life is more recent, so it isn't present in the older texts, unless some copyist decided to "correct" that. Book of Daniel is believed to be quite "recent" compared to the most of Old Testament texts:
Also "theologically" according to the Jews their (older) Bible simply mentions "everlasting life" in the "Book of Daniel" at least century before the birth of Jesus. But they believe that the texts about Jesus written and collected by that newer sect that called them Christians simply don't prove that he's Messiah, as they are often based on misreadings of the Hebrew Bible, see for the example how NT came to use "the son of man" expression:
I'm just saying you can easily reconcile Ecclesiastes with the Christian notion of heaven by considering its context. I don't know if there's any such problem to be reconciled in Judaism.
My response was always to your claim that "the possibility of eternal life in heaven was only offered by Christ" and all my responses were to demonstrate that it's not "only by Christ" but an older and very known concept. They weren't supposed to help you reconcile that fact with your favorite theology.