I have been stuck with it for over 3 days, is there any solutions to resolve that? Thanks in advance.
Group label not showing up in NodeBB vanilla theme
The group label wasn't added in to vanilla. I personally had a look to see where it could go, but with the first post and subsequent posts being different, it wasn't easy. If you want to add it to your own theme, the code is between line 48 and 54 here.
If you find a good place to put it that works reliably, maybe make a PR for anyone else that uses vanilla.