Magento 2 Product attributes are specific product properties used to describe a product as well as for other purposes like sales rule conditions.
If you configure sales rule based on a product attribute and cannot choose (don't see) the required attribute, please go to Admin Panel > Stores > Attributes > Product.
Then choose the attribute you are interested in and in the Storefront Properties section choose Yes in the Use for Promo Rule Conditions field.
Once you have finished don't forget to press the Save Attribute button in order to be able to successfully use this attribute in the sales rules.
One of the distinctive features of Magento is flexibility. It allows you to create different attributes and use them for different purposes like adding dynamic category rules or defining conditions for CMS blocks display.