We have confirmed there is an issue with the sub-categories not showing for selection. We have alerted the technical team and expect the issue to be addressed and resolved shortly.
In the meantime - You can create a product if you do not enter anything in the store information area of the product set-up page, however, the product will not show on the store.
Later, when this is resolved you can add the store information by editing the product.
We sincerely apologize for the inconvenience.