Hello Ahmed,
You are mapping an input structure of your BAPI ES_HEADER as output which is causing the error.
You need to map the out put table as the response while performing mapping.
Correct this and issue will be resolved.
Also note that while mapping in SEGW you cannot map more than one table as response.
only one table can be mapped as response in SEGW while using performing mapping option.
If at all you need to map more than one table as GW Response then in that case you need to create GW Model with different entities and association between them with appropriate cardinality and Create run-time objects.
Then manually implement Expand Entity/Entity Set method by calling that BAPI inside it in DPC_EXT class.
Refer -> Let's code association/navigation and data provider expand in OData service!
Also while implementing QUERY there is no rule that input must be mapped. Its always an optional.
Only rule is the Key properties of your GW Model need to be a part of out put mapping.
Regards,
Ashwin