This is what Trump himself confirmed here:
“Comrade Kamala Harris just informed us that she will not be participating in the Fox News debate on September 4th. I am not surprised by this development because I sense that she knows that it is very difficult, at best, for her to defend her record flip-flops on everything she once believed in, including her statements that fracking will not happen in Pennsylvania and her terrible performance at the border, where millions of criminals, people from mental institutions, and terrorists have been allowed to flow into our country, with absolutely no checks or scrutiny. It is called, and she loves it, Open Borders!!! In lieu of the debate on September 4th, she has agreed to do a televised town hall, hosted by Sean Hannity, for Fox. It will be held in the great Commonwealth of Pennsylvania – details to come!”
As previously mentioned, he will instead be doing a TV interview on the same date.