Currently I'm working on prplos which is built on top of the openwrt only.
Let it be consider as openwrt. I'm using my linux pc 20.04 Ubuntu version.
I build the prplos image on it which is in sqashfs filesystem. I connected the board to my pc. Followed the document to flash the image on it. After getting into bootloader.
But while using the untar command to extract that bin file it's throwing error like command not available.
The uboot version is 2016.01.06 (2020 release).
It appears you are using firmware that is not from the official OpenWrt project.
When using forks/offshoots/vendor-specific builds that are "based on OpenWrt", there may be many differences compared to the official versions (hosted by OpenWrt.org). Some of these customizations may fundamentally change the way that OpenWrt works. You might need help from people with specific/specialized knowledge about the firmware you are using, so it is possible that advice you get here may not be useful.
Ask for help from the maintainer(s) or user community of the specific firmware that you are using.
Provide the source code for the firmware so that users on this forum can understand how your firmware works (OpenWrt forum users are volunteers, so somebody might look at the code if they have time and are interested in your issue).
If you believe that this specific issue is common to generic/official OpenWrt and/or the maintainers of your build have indicated as such, please feel free to clarify.