...probably didn't exist.
There were many, many translations of the Old Testament scriptures into Greek. But "LXX" and "any Greek translation of the Old Testament" are not the same thing.
Throughout the Greco-Roman empire, Jewish synogogues would reasonably have their own translation of different Old Testament books. Practically everyone spoke multiple languages in the empire, so using one common tongue was practically necessary.
Koine Greek ("common" Greek), the language in which the New Testament appears, was invented specifically to help people across the Greco-Roman empire to be able to communicate in one language that they all had in common. Hence, the name.
So why say "The Septuagint" probably didn't exist?
Because this is what THE Septuagint is supposed to be:
Seventy different Hebrews were asked to make a singular translation of the Old Testament into the Greek language. (As though everyone didn't already have their own, and as though it was really necessary.) When they all emerged (magically at the same time), they all had the exact same translation, word for word, as each other. That is "THE Septuagint." Again, that's why it has the name it does ("Septuigent" means "The Seventy," and is why it is abbreviated with the Roman numerals for 70, LXX).
This should sound silly to anyone who has ever even met an academic.
There were definitely different Greek translations of the Old Testament. Just be aware that there were many, many such translations, not just one weird magical one that seventy guys made all at the same time. The different translations were made by many different scholars over the course of decades, and are no more inspired than modern translations are.
If you are having problems with some Biblical text, Greek translations of the Old Testament can be helpful, but try to not take them as authoritative. That is, they might give some insight in to how some people were thinking of a certain word or phrase, but it would be ill-advised to try to intepret the Hebrew Old Testament by using a Greek translation of it. It would be the same as using a modern English translation to try to re-interpret those same texts.