In case you missed it Biden as the sitting 1 term democratic president is the automatic nomineeThe first debate was between Biden (the presumptive Democratic nominee, note the description 'presumptive'), and Trump the presumptive Republican nominee, (again note the description of 'presumptive')
The 'presumptive' descriptor refers to the nominee who is expected to receive the 'declared' nomination of that party, KH is currently the presumptive nominee for the Democrats, so Trump has no legitimate excuse to refuse the debate.