Hi @kanchandebnath - welcome to the farmOS community!
The farmOS code is licensed under the General Public License version 2 (GPLv2).
This means that any code that is built on top of it must also be released under the same GPLv2 license. So if you create and distribute new features, they must be released as open source. This allows for the farmOS community to share and grow together, and we would not exist if we were not already building on the work of others that came before us (Linux and Drupal are both GPLv2 as well).
Furthermore, the name “farmOS” is a registered trademark, and acceptable usage of it is outlined in the farmOS Trademark Guidelines: https://farmos.org/community/trademark/
As to your question:
Is is legally allowed to Build Custom Application around FarmOS for Commerial Purpose, so that I can sell to my customers.
As long as you are abiding by the GPLv2 (which requires you to release any derivative code you distribute under the same license), and the farmOS Trademark Guidelines, then you are free to build features on top of farmOS and sell to your customers.
There are many successful businesses that have developed alongside open source software projects. In general their business model does not depend on selling proprietary licenses to the software itself (because the source code is free and anyone can copy and run it themselves), and instead they provide complimentary services.
I would love to see a larger community of third-party service providers grow around the farmOS software, providing services like support, training, hardware/sensor integrations, agronomic services, etc. None of these rely on hiding the source code and making money on licensing.
I hope that helps. I am not a lawyer, so please consult with one if you are uncertain. And I hope you find farmOS a useful tool and will help to bring it to more farmers!