There is not much issue with your creature. Most of the issues are explanatory more than anything else.

The split tail is not an issue split tails have evolved more than once in earth life (costly in marine animals), and if you look at an elephants trunk it is easy to see why such a structure might be useful, they basically function as fingers. Your creature will likely use its mouth as well as it really needs another limb for generating counter forces, the opposing "limb" does not need fine manipulators but it does need some strength and gripping ability. precisions angled impact using opposing forces is necessary to master flint napping which is the beginning of really all advanced tools. You can do alot with one hand as long as you have grippy anchor to oppose it. Unless I am reading its anatomy wrong the neck is fairly long so it should work fine.

The full size of the brain was probably not achieved until after the advent of fire and cooking as it was in humans (the hominid brain rapidly doubled in size after the discovery of fire) cooking food drastically increases its available caloric value.

Your real problem is why it has the tail in the first place, anatomically the animal is not that different than a ground sloth, so it likely evolved from a small climber, as it evolved larger size is altered the feet into something more elephant like loosing any use as grippers. It will need to be far more social of course and may even end up more social or at least more cooperative than humans, with the reduced grasper count. But you need a reason it kept the prehensile nature of the tail, that is going to be up to you, I suggest either some unique use in feeding, or some some kind of very strong sexual selection.

You will need a more fertile/productive cold climate region than earth, you need to support social groups year round prior to any technology, but it does not need to be that much more fertile. Just more efficient photosynthesis should be enough, and that's not too hard photosynthesis is not so efficient that more efficient means evolving elsewhere is unbelievable.

There is not much issue with your creature. Most of the issues are explanatory more than anything else.

The split tail is not an issue split tails have evolved more than once in earth life (costly in marine animals), and if you look at an elephants trunk it is easy to see why such a structure might be useful, they basically function as fingers. You could even have it be a common feature in a whole group of organisms on their planet, maybe the split serves some important developmental function so it is never completely lost in the clade. That gives as strong jumping off point.

Your creature will likely use its mouth as well as it really needs another limb for generating counter forces, the opposing "limb" does not need fine manipulators but it does need some strength and gripping ability. Precisions angled impact using opposing forces is necessary to master flint knapping which is the beginning of really all advanced tools. You can do alot with one hand as long as you have grippy anchor to oppose it. Unless I am reading its anatomy wrong the neck is fairly long and stout so it should work fine.

The full size of the brain was probably not achieved until after the advent of fire and cooking as it was in humans (the hominid brain rapidly doubled in size after the discovery of fire) cooking food drastically increases its available caloric value.

Your real problem is why it has the tail in the first place, anatomically the animal is not that different than a ground sloth or a chalicotherium. The only real use of a prehensile tail is in climbers so it likely evolved from a small climber, as it evolved larger size is altered the feet into something more elephant like, losing any use as grippers. It will need to be far more social of course and may even end up more social or at least more cooperative than humans, with the reduced grasper count. But you need a reason it kept the prehensile nature of the tail, It is a lot of tissue and neural support it will need a strong selective pressure to keep it around. The exact reason is going to be up to you, I suggest some kind of very strong sexual selection or something similar or perhaps some unique feeding mechanism. Because it is an alien organisms perhaps it serves a purpose not seen in earth vertebrates, like as a mating organs or some vital use in child rearing.

You will need a more fertile/productive cold climate region than earth, you need to support social groups of these organisms year round prior to any technology, but it does not need to be that much more fertile. Just more efficient photosynthesis should be enough. That's not too hard photosynthesis is not so efficient that more efficient means evolving elsewhere is unbelievable.