She can’t articulate why the Fox debate shouldn’t happen. It’s a classic negotiating technique, put Harris on the defensive and make her see things your way. Then just like that Kamala will start feeling overwhemlmed and suggest the the very deal Trump was looking for all along: a debate on Fox and multiple debates.
Of course she doesn’t want to debate (loosely used) on Fox. She will feel much more confident of doing well on ABC since she will get all the questions beforehand and will be handled with kid gloves. Trump will be foolish to accept her terms, although I’m sure the RINO’s around him will urge him to do so.