This error is coming from RunUAT.bat in Engine/Build/BatchFiles. The script is looking for Intermediate/Turnkey/PostTurnkeyVariables.bat, but it doesn’t exist there (or anywhere in the UE5 file system). I’m not sure in what case you need this batch file, as I don’t use Turnkey to manage my sdks. In any case, this warning is happening because the script is wrong: you need to wrap the path after if EXISTS in quotes.
If you want to suppress this warning (which seems safe to do), at line 82, make this edit:
if EXIST "%SCRIPT_DIR%..\..\Intermediate\Turnkey\PostTurnkeyVariables.bat" (
rem ## We need to endlocal so that the vars in the batch file work. NOTE: Working directory from pushd will be UNDONE here, but since we are about to quit, it's okay
endlocal
echo Updating environment variables set by a Turnkey sub-process
call %SCRIPT_DIR%..\..\Intermediate\Turnkey\PostTurnkeyVariables.bat
del %SCRIPT_DIR%..\..\Intermediate\Turnkey\PostTurnkeyVariables.bat
rem ## setlocal again so that any popd's etc don't have an effect on calling process
setlocal
)