The US Gov under its agreement with China (PRC), recognizes Taiwan as part of China. Refer to the video below from Jimmy Carter who, as President, read the agreement out - in particular 2 minutes onward. And as part of that agreement, there would be no official contact with the island of Taiwan.
Now, if Nancy was a retired politician as Pompeo (who visited Taiwan a few months ago), that would be ok.
But she is Not. She is third in line to the US presidency. Here official visit (without getting permission from China first) would constitute an official recognition of Taiwan as a separate State from China.
