Ah, ok then. There are a couple of things going on.
The ‘copy’ ActionScript command will not overwrite an existing file. In your case it looks like “Custom-Medium.otf” already exists in the C:\Windows\Fonts folder. You’d have to delete it from there before you can copy over the new one.
Also, for new fonts, just dropping the font into the Fonts folder is not enough to “install” it for Windows. There were scripts I’ve used to register the fonts with Windows in the past, but a quick google makes it look like some things have changed. Have a look at https://jordanmalcolm.com/deploying-windows-10-fonts-at-scale/ and see whether that’s helpful at all.