I’m really sorry to hear that your animal passed away
an unfortunate reality of keeping these animals in an aquarium is that they just don’t live for all that long, especially if they were collected from the wild as an adult. It’s always hard to lose a member of the family.
Aside from doing a full autopsy, the only thing you can really look at is the state of the body when it died. Was it missing any arm tips or arms entirely? Was it’s mantle receded and curled up in the back where the siphon comes out?
Don’t let this situation discourage you from trying again. Most cephalopods live for less than a year and a half so yours very well could have been collected near the end of its life. If your water parameters were all within range and you fed the animal when it was hungry and it had a normal light cycle, then you did all you could. Hopefully you learned a few things and you will have an ever better experience with your next octopus!