I've been doing research for my speculative evolution project, and I am currently mapping body plans for the main lineages of organisms I'm going to feature in this project. One of them has an endoskeleton derived from subdermal armor structures and I was wondering whether it would be possible for it to have an endoskeleton made of cellulose, or chitin, or some kind of combined hybrid material of those two. Of course it wouldn't be pure chitin/cellulose and there would be proteins and such incorporated into it to alter its quality, but I think it's an interesting question nonetheless. This idea came from the fact that cellulose and chitin are similar in their structure. For reference, this spec evo project is on a mostly Earth-like planet with the differences being a binary red star system and a planet with 1.5x Earth's gravity.
Cellulose and chitin are both hard materials, but are inferior to bone when it comes to building an endoskeleton. They are softer and offer less support than bone - the trade-off being that because they are a little flexible, they can handle impacts that would chip or dent bone.
This is why chitin, rather than bone, tends to be used for exoskeletons, since they are on the outside of the body where they are more susceptible to direct impact. Bone is shielded with layers of fat and muscle, so it can be harder and more brittle.
That doesn't mean it's impossible for a creature to have a chitinous endoskeleton, but it would be weaker than a bone one. And on a planet with higher gravity, you probably want skeletons to have more strength, not less.