Hi Girdhari,
let me do an assumption 
. Did you create a entity type+entity set, generated the artefacts, and did the "Map to Datasource" and did you then change something in the entity set and tried to "re-generate"? If yes, can you please try the following workaround:
1. Delete the entity type, the entity set, and the service implementation
2. Save the GW Project and restart the GW Service Builder Transaction (/nsegw)
3. Create the Entity Types/Set again (via the "Import DDIC Structure") etc.
4. Repeat all the necessary steps from the E2E guide, i.e. Map To Datasource, Generate, ...
Let me know if the issue persists.
Cheers,
Jasmin