Are you launching the IDL VM, and then selecting the .sav file from the file selection dialog when you get this error? On windows, this can be found in the program group under IDL 8.5->Tools.

Yeah! Let me describe the procedure I used. Through launching IDL VM from the start menu, it opens and prompts me to "Click To Continue". Then it prompts me to "Please Select the IDL save file". Through selecting the .sav file, it now prompts me to "Please Select a File to Read" (showing .CSV as the default file name extension). Now when I select the .CSV file, I receive the mentioned error message saying: "Warning.. Attempt to subscript TT with ll is out of range". I am using Windows 10. After launching the IDL VM, all actions were executed in a Windows Explorer environment (window) while I expected the opened .sav file to launch a GUI through which I could open the .CSV file for statistical optimization purpose. PS: I don't have a (full) licensed IDL developer version activated on the machine. I am just enjoying the IDL VM free use functionality.

That sounds like an error in the code and since it is compiled in the .sav, you would need to contact the developer of the code. I think the VM is working as expected. It may be that the .sav was created in an earlier version of IDL and may not be compatible with the latest VM.