Can't write to registry

My setup program created three keys in the Win 10 registry and I can read them but when I try to change the value of one of them I get an error 5 (lack of access priviledge). My code looks something like:

Subroutine ChkReg()

use ifwinty
use user32
use advapi32

implicit none

Integer(4):: iret, iLen
Integer(4):: lret
Integer(4):: hKey = 0
character(255):: buff

    buff = ""C
    iLen = sizeof(buff) - 1
    iret = RegOpenKeyEx (HKEY_CLASSES_ROOT,     &"PDC"C,                                                       &
                         0,                                                                    &
                         KEY_EXECUTE,                                           &  ! only key that works
                         LOC(hKey) )

    if (iret == ERROR_SUCCESS) Then
       buff = ""C
       lret = RegQueryValueEx( hKey, "Data1"C, NULL, NULL, &
                               LOC(buff), iLen )
       if (iret /= ERROR_SUCCESS) Then
          buff = ""C
          lret = RegQueryValueEx( hKey, "Data1"C, NULL, NULL, &
                               LOC(buff), iLen )
          iret = MessageBox(NULL, buff, "Data1 Register PDC2", IDOK)
       else
          iret = MessageBox(NULL, buff, "Data1 Register PDC1", IDOK)
       end if

       Buff = "123456789"C
       iLen = 10
       iret = RegSetValueEX ( hKey, "Data1"C, 0, REG_SZ, LOC(Buff), iLen)

       iret = RegCloseKey(hKey)

    End If

End Subroutine ChkReg

All works well except the RegSetValueEX which returns an error code of 5. If I try a key of KEY_ALL_ACCESS then I get an error code 5 in RegOpenKeyEx. Any suggestions will be much appreciated.

Thanks,

Brooks