How can Trump supporters attack China for downplaying the threat of the virus and claiming they had it under control but ignore the fact ?

First of all, Trump would not know the truth if it smacked him in the face.  In fact, the only way we know Trump is not lying is when his lips are not moving.  Trump says whatever, whenever, without any regard for truth or falsity.  What is sad is that many of his followers are ignorant, presumptuous, disdainful faux Christian bumpkins, who lap up every word is he saying.  Why do they follow him?  Because he is white and male, and most of all, he is not Hillary.

In addition, Trump praised China for their handling of the virus.

Let's see....CHINA had the virus. CHINA was told how bad their security was back in November and did nothing. CHINA allowed the virus to get out. CHINA disappeared doctors who wanted the world to prepare. CHINA continued to lie while they stockpiled PPEs that should have been shipped to the US. CHINA stopped Wuhan residents from moving within China but continued to allow them to travel internationally, spreading the virus to Italy, Spain and the US. 

Trump closed the borders to the Chinese 2 DAYS after CHINA finally admitted their was a problem, on JANUARY 31ST. Libs like Pelosi, Kenny of Philadelphia and his health commissioner, Cuomo, DiBlasio and his health commissioner were calling him a racist while telling people to party in Chinatown.

FEBRUARY 16, 2020 she tweeted, "As a doctor I can say there is no reason to avoid ANY neighborhood because of the coronavirus. As a New Yorker, I know that ANY discrimination is abhorrent and unacceptable." [A comment she made when she discovered that some NYC cab drivers were avoiding Chinese neighborhoods to avoid picking up Chinese passengers.
FEBRUARY 16, 2020 Mark D. Levine Chair of New York City Council Health Committee tweeted, "In powerful show of defiance of coronavirus scare, huge crowds gathering in NYC's Chinatown for ceremony ahead of annual LunarNewYear parade. Chants of "be strong Wuhan!" If you are staying away, you are missing out! [along with his tweet are photos of the huge crowd]
MARCH 02, 2020 Bill de Blasio - the MAYOR of New York City tweeted, "Since I'm encouraging New Yorkers to go on with your lives + get out on the town despite Coronavirus, I thought I would offer some suggestions. Here's the first: Thurs 3/5 go see "The Traitor" @FilmLinc. If "The Wire" was a true story + set in Italy, it would this film.

Uh... were you born yesterday?

Dr Li Wenliang Spoke out about the virus and the CCP had him admonished. CCP made doctors and journalists disappear when they started to tell social media about the virus.

Let us not pretend democrats were not also downplaying the virus situation......... 

Trump believed or hoped that it was true, and at first what he believed about the epidemic was based on what China had reported to the WHO. 
He only stopped believing China and pivoted to getting information from his own task force after patients in the USA were proven to have passed the virus to others through casual contact. (something that China said couldn't happen.) 

Long before that, the Chinese government knew how contagious the disease is, and shouldn't have misled the WHO, and by doing so, misled Trump and the rest of the world.

Because according to statistic, trump supporters are more likely to be white men with only high school degree. And those people are more likely to be swayed by all the lies on the media.

Trump is only guilty of believing China.

You have a good question but the debate is endless.

I had unfortunately did the same thing.  Because I don't remember anything coming about with the Bird Flu or SARS.