To mount internal storage you just need to run two commands.
First open new Termux session (Swipe from left corner to right) and type:
termux-setup-storage
Now it will ask you for Storage permission. Allow the Storage permission.
Once you are done with it type:
nano start-{distroname}.sh
Make sure to replace {distroname} with your specific distro name.
Now search for line:
#command+=" -b /sdcard"
If you find this line then remove it and change it to
command+=" -b /sdcard"
If you don't find the line then add the above mentioned line as shown in the screenshot below.
Once you are done with it press Ctrl+X and then press Y and then enter. Now start your linux system and do cd /
. Now you will be able to see a folder sdcard which means you have successfully mounted your Internal Storage.