The app's homepage at http://digitalsquid.co.uk/netspoof/ lists the requirements needed for the app to work:
Requirements
Network Spoofer runs on Android devices with the following requirements:
- Which are rooted (‘su’).
- Have an SD card (or internal storage) with >450MB spare space.
- Which has Wifi
So I'd start by double-checking how much free space you have on your device. Then it lists the currently supported (and unsupported) models:
Supported devices
Currently Network Spoofer has been thoroughly tested on these devices:
- Samsung Galaxy Nexus
- HTC Nexus One
- T-Mobile G1
It is reported working on these devices, as well as working on most
other devices:
- LG Optimus 2x
- Huawei u8150d
- HTC Hero
- Droid Bionic
- Samsung Galaxy S / S II
- Samsung Infuse 4G
However, Network Spoofer doesn’t currently work with:
- Droid / Droid 2 / Droid X
- Motorola Atrix
Many other devices should and probably will work, but please
report any odd behaviour or crashes, so I can get this working
with more devices.
Your device unfortunately isn't on the list of currently supported devices, which means that the app's author doesn't guarantee that it will work for you.
If you follow the bug reporting link there are ways there for you to provide the dev with more information, so that they can try to get things working on your device. But before you go there, have a look at Network Spoofer's FAQ page which does list some common problems, and some ways to get round them, or what additional information you can provide to help get the problem fixed.
Finally there is currently an open bug report about Network Spoofer not working on an S4, with the same error message that you're getting, that does list how the reporter of the bug got it to work:
I had the same problem when trying to run bt5 on my phone using the
Complete Linux Installer app, but solved the problem by changing the
script from the file bootscript.sh (attached) to the combination of
the 4 other files (installbt first, then startbt, and bt, then stopbt
to unmount everything). The chroot command is located in the bt
script.
That could be worth trying (you'll need to visit the bug report to see the attached files).