Clients using a POS Integration do not have a concept of products/inventory in your MyBlackbird account. Because of this, you are not able to assign any products to a 'Product Type' reward.
To work around this, you can simply create any other reward type and ensure the reward name is descriptive enough to indicate it's a product reward.
In place of a product reward, feel free to create one of the following types of rewards:
Brand - A reward specific to a brand like 'Dosist'.
Category - A reward specific to a category of items like ‘flower’
Order - An order-specific reward like ‘$10 off $40 or more’