Hi Hary_Singh and welcome to WebmasterWorld [webmasterworld.com]

The Popular Products section of the serps is part of Google Shopping, so if you have not yet set up your Google Shopping account, that would be your first step. You would submit feeds containing product information to Google Shopping and that account interface would show you what information is required. It is up to you to supply complete and accurate information that is updated with any changes to your product feed. If you have already taken those steps and your products are not showing, then take a look at the accuracy of your submitted information.

Google offers more details here: [support.google.com...]

I have set up merchant center already. Also submitted feeds into regular basis. But still not showing.
Give me any solutions or reference where I can learn

Google offers more details here: [support.google.com...]