Italic JavaScript that converts regular text into italics

Make sure that that your character style's name is really Italic. There could be an invisible character in the style's name. I've seen quite a few cases of a style name that had a return at the end or a tab. You don't see that in the panel, but you can try this: apply the style to some text, then click somewhere in that text and run this script:

app.selection[0].appliedCharacterStyle.name.length

In your case it should return 6.

The problem can happen if you (or some other user) copied a name and used that to name the style.

To fix a problem like this, simply re-type the name in the panel.

P.

In addition to what @brian_p_dts mentioned also make sure that the style is not a part of a group. You could even try the following code snippet

app.selection[0].appliedCharacterStyle=app.documents[0].characterStyles.itemByName("Italic");

For example on how to deal/find style within groups you can refer to the following discussion. It talks about paragraph styles but the code for character style also remains the same mostly with changes only to the name of property and methods.

https://community.adobe.com/t5/indesign-discussions/script-find-paragraph-style-under-the-folder/m-p/12833881#M470607

-Manan 

You sure the document has a Character Style called "Italic"?