I have a question about configuration of computer's block producer(BP). My question is: In 21 BPs, if have some BPs which have lower configuration of computer than other, they will never be approved longest chain, so is this error?
It is not the case. In EOS which has a DPoS consensus model, BPs work together to produce blocks and forks are not very often. The 21 active BPs will take turns to produce blocks one by one. As long as a BP can produce blocks as expected in its turn, everything is fine. It does not need to compete with someone else as if it's in the Proof of Work consensus model.
If a BP server has a lower specification, it may take more CPU time executing all transactions. And if it takes too much time, the blockchain will enter congestion mode, in which CPU resources will look more scarce for every user.
ref: https://trybe.one/breaking-news-proposal-to-increase-trigger-cpu-congestion-mode/ this article on CPU congestion mode