Thank you a lot for your help.
I tried to call osk.exe and gave the full pathname c:\windows\system32 in the option of system exec.vi
=>> it doesn't work
I copied osk.exe from c:c:\windows\system32 to my current folder, called osk.exe and specified the pathname option c:\blablabla
=>> it doesn't work
I copy osk.exe from c:\windows\system32 to my current folder and call osk.exe with giving the full pathname c:\blablabla\osk.exe
=>> it works