Hi Dhananjay Kumar,
I am Dave, I will help you with this.
When you re-install a PC that had Windows 10 pre-installed, that licence is stored on the motherboard, and there should be no need to use Command Prompt to reactivate Windows, that should happen automatically.
1
Open the Settings App, then go to Update and Security - Activation
What is the exact activation error shown on that page?
Is the newly installed edition (Home, Pro...etc.) of Windows 10 the exact same as was previously installed on that PC?
2
Where did you purchase your PC, was that from a PC manufacturer or from a PC Shop and was that new or used when you purchased it?
3
Click your Start Button, type cmd then right click Command Prompt and choose 'Run as Administrator'
Run or paste this command and hit Enter:
slmgr.vbs /dlv
Please post a screenshot of the resulting dialog