It's not true of course. Once again, he's just trying to cover for his incompetence in dealing with the virus. PolitiFact ruled Trump's latest statement as "False", saying the following:
Trump said that 99% of all COVID-19 cases are totally harmless. The White House provided no supporting data.
Looking at the worst possible outcome, the government’s numbers show a cumulative death rate of 4.5%, although the actual fatality rate could end up lower once we learn more about the disease The rate at which new cases lead to hospitalizations has fallen, but it still remains at 4%. And public health researchers note that more and more, there are reports that even people with mild cases are hit with medical problems after they’ve recovered. There’s a lot more to be learned about this disease, but nothing says that it’s harmless for 99% of the people it touches. We rate this claim False. Sadly though, most of the Republican base believes Trump over scientists and medical professionals. The chart below is from the Axios / Ipsos Poll (done between March 13th and June 29th of 15,871 adults, with a 2 point margin of error).