I'm using the theme "Metropolis". It allows you to select templates and sidebars within each page. I have installed the MarketPress plugin and configured it, added categories, and a product. The "store" page looks fine but all other pages within market press are missing a sidebar.
I have looked through a few threads here and tried the copy single.php page and rename to mp_product.php This at one point did add the default sidebar but made it under the product content (not floating to the right like a normal sidebar. As well it removed the Price, Category, quantity and ad to cart button.
It seems one way or the other code is missing.
I'm willing to send login if required.