I can't find product types UI I'm trying to add a new product type, but I can't find the tab for the product types.

In Manage product structure, it is very clear where that tab is, but I cannot find it.

I'm using Drupal 7.59 and Commerce Kickstart 2 with demo.
https://docs.drupalcommerce.org/commerce2/user-guide/products/manage-product-structure contains documentation for Commerce 2.x, which is based on Drupal 8.

You're using Drupal 7, which is Commerce 1.x, so you need to find and follow the Drupal 7 documentation. That should be as simple as clicking "Commerce 1.x" in the left-hand navigation panel.