How can I open the category menu in the sales window

On phone - swipe from right to left side.

Hello, 

You can find all your categories at the sales screen, under the "All items" button

If you click in the name of the category, you will see only the items that belong to this category. 

If you are using Tablet, you can arrange your items and categories on the sales screen of Loyverse POS  in a suitable way by creating new custom pages and organizing the items there. You can learn all details here: https://help.loyverse.com/help/how-arrange-sale-screen-loyverse