You can't. You should be able to, and you can with other symbols, but not SPX. Probably not with any other index either.
Basically, SPX is not a tradable instrument. It may have tradable derivatives, but as an index, its just a mathematical calculation which is updated periodically. SPX has zero volume of its own. Therefore, if you enable the volume subgraph, and pull up a chart with SPX, you will be met with an error informing you that volume has been hidden.
Further more, if you apply a script to SPX which includes the Volume() function, it will cause the script to fail. It does not matter if the volume function is a reference to a different symbol's volume, like
Volume("/ES")
, it will still fail. Below I've written a simple label to display /ES's volume from another symbol. With SPX, It doesn't come up as even 0 or NaN, it just throws an error.
I don't understand the reasoning behind this myself, perhaps its a bug in TOS/ThinkScript...
Open Interest will fail too, if I remember correctly.
This is the culprit behind those complex options-to-chart scripts not working with SPX options.
On a side note, there are some chart providers that pretend SPX has volume by displaying the total volume of all S&P500 components, or something like that, but its fakery as far as I know.