in a sub flow there is no price book field (in set input value)

On the 1500kW path, add a Subflow element:

• Referenced Flow: Subflow-Create Generator Opp Product
• Label: Create 1500 Opp Product
• API Name: Create_1500_Opp_Product
• Include the OpportunityId field and select $Record > Id
• Include the PricebookID field and select Price Book from Get_Pricebook > Id
• Include the ProductKW field and enter 1500

here there is no option for pricebookID field ,what should i do?