Hello Ganasri Mistry,
Thank you for posting in Microsoft Community forum.
To activate Windows 10 Enterprise N LTSC, you will need a valid product key for that specific edition. You can purchase a product key for Windows 10 Enterprise N LTSC from Microsoft or an authorized reseller. If you are unable to activate your current installation with a normal Windows 10 LTSC 2019 product key, it is likely because you have installed the N version, which requires a different product key.
As for why you are unable to install the regular Windows 10 LTSC 2019 on your Dell Optiplex 7010 tower computer, it could be due to compatibility issues with the hardware or drivers. You may want to check with Dell to see if they have any specific recommendations for installing Windows 10 LTSC on that particular model.
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Haijian Shan