Why can't I save apps into my SD-card?
You have to wipe your cache partition. It is the partition where Android stores downloaded .APKs and some other system data and you cannot see it with a file manager.

Step 1

Turn off your Android device. Reboot the phone or tablet into Recovery Mode. The process is specific to the device model. For Lenovo S750:    


Press and hold Volume Up and Power Button at the same time.
Release all buttons when the Lenovo S750 screen flickers and in a few seconds the Recovery screen will appear. 
When the Android image appears press Power shortly


Step 2

Press the “Volume-down” button to scroll to “Wipe Data/Factory Reset.”, select it.

Step 3

Scroll to and select “Wipe Cache Partition.”

Step 4

Highlight and select “Reboot System Now.” The phone restarts.

DO NOT make "Factory reset". You will loose all your data on phone.