Environment variable not recognized [not available] for [Run] programs in Inno Setup

The processes created for executing entries from the [Run] section inherits the environment block of its parent process, which is the installer itself. So you have to set the environment variable to the installer and let it inherit to your executed application. How to do that is shown in the below script:

[Run]
Filename: "{app}\temp\installation_files\license.exe"; BeforeInstall: SetEnvPath

[Code]
#ifdef UNICODE
  #define AW "W"
#else
  #define AW "A"
#endif
function SetEnvironmentVariable(lpName: string; lpValue: string): BOOL;
  external 'SetEnvironmentVariable{#AW}@kernel32.dll stdcall';
 
procedure SetEnvPath;
begin
  if not SetEnvironmentVariable('PATH', ExpandConstant('{app}')) then
    MsgBox(SysErrorMessage(DLLGetLastError), mbError, MB_OK);
end;

---

Previous answer for notifying rest of the system about variable change:

As @Jerry pointed out in his comment, a notification about the environment changes is performed after the [Run] section is processed. Actually, it is one of the last things executed by the installer.

So, to notify the system about environment changes before processing the [Run] section, you’ll need to have a workaround. I rewrote the RefreshEnvironment procedure from Inno Setup code to script. It’s the same function as it’s executed if you have ChangesEnvironment directive set to yes.

In the following script I have removed the ChangesEnvironment directive and added execution of the RefreshEnvironment procedure from the AfterInstall parameter function of your registry entry:

[Registry]
Root: HKLM; \
    Subkey: "SYSTEM\CurrentControlSet\Control\Session Manager\Environment"; \
    ValueType: string; ValueName: "PATH"; ValueData: "{app}"; \
    AfterInstall: RefreshEnvironment;

[Run]
Filename: "{app}\temp\installation_files\license.exe";

[Code]
const
  SMTO_ABORTIFHUNG = 2;
  WM_WININICHANGE = $001A;
  WM_SETTINGCHANGE = WM_WININICHANGE;

type
  WPARAM = UINT_PTR;
  LPARAM = INT_PTR;
  LRESULT = INT_PTR;

function SendTextMessageTimeout(hWnd: HWND; Msg: UINT;
  wParam: WPARAM; lParam: PAnsiChar; fuFlags: UINT;
  uTimeout: UINT; out lpdwResult: DWORD): LRESULT;
  external '[email protected] stdcall';  

procedure RefreshEnvironment;
var
  S: AnsiString;
  MsgResult: DWORD;
begin
  S := 'Environment';
  SendTextMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0,
    PAnsiChar(S), SMTO_ABORTIFHUNG, 5000, MsgResult);
end;