While initial reports indicated that customers in China were encountering errors when trying to add cards to Apple Pay, it seems that this was intentional on Apple's part. The company was evidently enacting a staged rollout of the service across China, which led to some customers being unable to add their cards right away. This had initially been reported as an error due to strain on Apple Pay's infrastructure.
From 9to5Mac:
Apple has since informed us that the statement by a local representative was not an official statement and has been mistranslated from the Chinese by Caixan. The correct information is that the ability to add cards was being made available on a rolling basis throughout the day.
According to Mashable, about 38 million cards had been registered with Apple Pay by 5 p.m. China Time on Feb. 17, with 10 million cards added in the first hour.
Apple Pay official launched in China on Thursday, Feb. 18. The launch comes thanks to a partnership between Apple and UnionPay, China's domestic bank card provider. China is the fifth country in which Apple Pay has launched, following the U.S., UK, Canada, and Australia.