One of the features embedded into the new Galaxy S7 device is the Energy Saving (Power Saving) Mode. It is similar to the power saving features found in prior Galaxy devices. When enabled, this feature will automatically disable the touch keys on the phone as one way to conserve battery power. This then results to the touch keys not lighting up on your device
That being said, the only thing you need to do to make your phone's touch keys illuminated again is to exit or deactivate the Energy Saving Mode. In case you need help on how to access the right settings menu for you to configure, I have provided the following steps for you to refer to.
Steps to disable power saving modes on your Galaxy S7:
1. Tap Apps from the Home screen.
2. Tap Settings.
3. Scroll to and tap the Battery icon.
4. Tap Power Saving Mode.
5. Toggle the On/Off slider to turn it OFF.
After disabling the Power Saving Mode, you should then see the soft touch keys are illuminated again.
To access display settings on your Galaxy S7/S7 Edge smartphone, go to Apps-> Settings-> Display-> look for Capacitive Keys (Touch Key Light Duration) or similar option. Try to set this option to ALWAYS ON, and then set Power Saving Mode to IMMEDIATELY.