Taiwan is not in the UN because several reasons, first of all because the name of that country (the official name) that is: Republic of China.
this name comes since china went democratic in the 1911, but after a civil war china changed from democratic to communist. so the government of that time was on Beijing and that government was Democratic so they move all the people to what's now Taiwan that in that time that land belong to China. and they made that because they think that from Taiwan they can expand democracy but they couldn't.
So China doesn't want that Taiwan get to the UN because with that they became a Nation State but they should put the name of republic of china but they couldn't be 2 chinas on the world.
