Hi,
In the backend, when the window width is not enough to display properly all the columns of a listing, HikaShop has CSS to automatically hide the less useful columns, starting with the ID column.
So if you're looking at the backend in a window that isn't fullscreen, change it to be fullscreen and you'll see the ID column.
Otherwise, it could be that you're looking at the page with a monitor with a low resolution. Increase the resolution if possible.
Last solution is to remove the CSS:
table#hikashop_product_listing tbody td:nth-child(1),
table#hikashop_product_listing th:nth-child(1),
table#hikashop_product_listing tbody td:nth-last-child(1),
table#hikashop_product_listing th:nth-last-child(1),
in the "backend CSS" setting under the Display tab of the HikaShop configuration in order to keep the ID column even when the width goes below 1280px.