You have to choose MAIN before you can actually split those tokens. What you can do is click "Split", get a "Split" address by copying it from "Receive". And then log into MAIN, and SEND your BCH over to the address you copied, by pasting it in the SEND tab.
That will put it in a separate chain altogether, which is what is recommended. You need to get access to those coins on the main chain, basically, first.
RE: How To Access The Bitcoin Cash (BCH / BCC) On Your Ledger Nano S