I see. This isn't supported by Product Levels at this time but I've just tagged this thread as a "feature suggestion" , so if we see enough interest from our community, we'll consider adding support for this in the future. But let me say that this way of working is not very common. You can have variations but, usually, you can have stock for one of the variations but not for others, and managing the stock by the parent, doesn't allow you to do so. Of course, that there are cases where this does not apply, I know, but just saying that is not common.
For now, I'd suggest you manage the stock at the variation level if you need to use Product Levels. Anyway, if you enable the BOM stock control option, all your variations can share the same BOMs and if one BOM runs "out of stock", all your variations will run "out of stock" at the same time, so the final behavior is similar to managing the stock at the parent level.