Baked sweet potato skin hard as a rock

It could be that your oven temp is too low during the cook, so you are essentially dehydrating the outside of the potato. I would preheat to 350 for at least 30 minutes, to make sure your oven is up to temperature. Then put the potatoes in.

Potato skin does not soften in the oven when cooked this way - it just isn't a thing that happens, it cannot be expected to.

The interior of the potato will soften as it gets cooked (with the moisture already present in the flesh interacting with the starches), but the peel is a protective layer - it starts out tougher than the flesh. Then, when it is baked, it also has much less exposure to the moisture inside the potato, and is on the other side exposed to air that is not only hot but dry, so it dries out and toughens right up.

This is expected in a baked potato. The classic baked potato has their peel generally used as a bowl, it gives structure to hold the potato innards and toppings together while being scooped out and eaten. Baked sweet potato is the same, the flesh is scooped out of the shell of the peel, or the peel tugged off the flesh in stiff layers - the peel is sort of sacrificed to get the interior cooked nicely.

Baked potato or sweet potato in just their skin is actually similar in effect to wrapping food in foil or parchment, just that skin is conveniently already attached. The peel is used to protect the interior, as you saw it can even get somewhat burnt without drastically effecting the edibility of the flesh, but as a result it usually doesn't end up palatable unless one takes steps to specifically make it that way.

If you add oil or fats, it still doesn't really soften, but will rather crisp up instead. Which is nice, but a different texture than you seem to be expecting. When crisping, the oil helps keep some of the moisture in the peel, and you can also use higher temperatures for shorter amounts of time - as the oil helps conduct heat - so the skin doesn't get as dry. Both will keep it from getting so tough. Even so, you probably couldn't cook a whole potato or sweet potato without the outside getting way overcooked before the inside's done, the technique works better with smaller pieces like potato wedges or fingerling or baby sized potatoes.

If you want the skin to soften, you need moisture - a bit to keep the skin from drying out while cooking, and more to have the spare moisture to interact with the heat and soften the skin. You can steam or boil, on stovetop, or if you still want to bake you can do so in a water bath, or in a sauce (cut up, for even cooking).

Or if you bake it for less time (at higher temp), so it doesn't get harder and harder again, and if the skin is on the more delicate side to begin with, you might be able to soften the skin up somewhat after baking by adding more moisture - water, or milk, or sauce - to counterbalance the dehydration in those outer layers. Though results will depend on the qualities of the skin, the specific variety, the time cooked, size (by the square-cube law), amount of moisture present in the potato to begin with, and probably a couple other factors.