Throughout the year, Trump begged to debate Biden with complete indifference to the details. However, the former president and his team became reluctant to face Kamala Harris as soon as she became the Democratic candidate. Trump fears how he would perform against a younger and more dynamic figure as Harris, so he is backpedaling now.
It doesn't matter how hard Democrats and their media allies try to act as if Trump is dodging the September debate; that claim will never be valid. He had agreed to face off with Biden twice after details were negotiated between their teams, so new negotiations had to occur after Harris' nomination. If she refuses this debate proposal, then she's the one who's afraid to debate.