Error exceptionMessage: Entity without screen binding cannot be used as top level entity.

Hi @ashishCodecorners, From the screenshot, I assume you trying to extract the Customer list from the Discount screen. As you can see in the below screenshot, DiscountCustomerDetail object is child entity of Discount entity. Also, the entity name of the DiscountCustomerDetail object is Customers. 

In the case, you need to include the Top level entity Discount in the request and expand the detail section using entity name Customer and not the object name DiscountCustomerDetail . Below is the request URL you can try,

• <<Your Instance URL>>/Entity/Default/22.200.001/Discount?$expand=Customers

If you like to filter a specific Discount, you can use filter like below,

• <<Your Instance URL>>/Entity/Default/22.200.001/Discount?$filter=DiscountCode eq 'DOC00C000'&$expand=Customers

I hope that helps.! To read more about the parameters in the request, refer, https://help.acumatica.com/(W(2))/Help?ScreenId=ShowWiki&pageid=c5e2f36a-0971-4b33-b127-3c3fe14106ff