At 35 cycles it gives 95% chance of a false positive. So the virus may not even exist anymore, it also has never been isolated.

There are more factors in which PCR can go wrong:
They do not do the essential negative test, to see if they caught something like the cold or the flu instead. This depends on the exact RNA bits they are looking for in which lab. They may just find a cat-virus or similar.

Additionally small contaminations can influence the outcome, and process errors during the cycles.