What did China do wrong in the beginning?

I don't understand why everyone is saying China tried to cover up the virus in the beginning. China reported pneumonia in 44 patients to WHO in December. China let WHO know that they were uncertain what was causing the virus and that all the cases were linked to a Wuhan market. China had already closed the market and was disinfecting it. Even WHO noted that there would be nothing remarkable about 44 cases of pneumonia (with an unknown origin) at that time of year, except that all the cases had originated at the same market. For this reason, it was not considered by anyone to be that serious at the time, and it was thought to be some local event. At the same time, China was working hard to determine what had caused these cases.