Awesome, I found it using the GetWritePath() command!
Here it is:
/home/asit/.config/AGKApps/NAU
Here "NAU" is the name of my project (an abbreviation actually). So, what I gathered is that this returns the project folder inside the home's hidden ".config" directory. To me it makes more sense to have a hidden ".agkapps" directory under my home folder instead of ".config" (that is for configurations in Linux), but it is what it is.
Also if you see the below image, the code did work and a file "app.sav" was written. I checked the file it does have content the game wrote in it.

I am making this project as an Android game so hoping it works on Android too. But I'll figure that out when testing on my device.
Thanks Virtual Nomad for helping me. Really appreciate it!