Permission denied on /system/xbin/su when trying to unroot Sony Xperia L

I'm trying to root my Sony XPeria L reading this tutorial The firmaware is 15.0.А.2.17, model C2105, and my Android version est 4.1.2 Everything was good until step 9, where I get the following error : /system/bin/sh: /system/xbin/su: Permission denied So I ran adb.exe from my computer and try to grant my permissions on this file. I tried some chmod 777 su but this is a "Read-only file system"... I'm a C# developper and I always used Windows, so Android world is totally new for me. Can anyone help me to avoid this error message ? Thanks you !