@ANKIT GHENGE Thanks for reaching out. It sounds like you're encountering a common issue with API mocking. When APIs return a 404 error after enabling mocking, it usually means that the requested resource is not found.
If you provide more details about your specific setup effective policy and the exact error message you're seeing, I can help you troubleshoot further. Have you checked the mock configuration is setup for the operation/API thats being requested?
https://zcusa.951200.xyz/en-us/azure/api-management/mock-response-policy
do let me know incase of further queries, I would be happy to assist you.