Cant check in with UA mileage plus

If you check in and choose United Airline frequent flyer number which the app allows, it always tells you that your number is wrong. I guess it only accepts numbers, not letters. How stupid! I wonder if they know anything about airline business. Looking at their website, it is also an amateur job. It is so shameful for the No. 1 airline in China. 值机常客卡号似乎只认识数字，不认识字母（比如美联航），太业余了。

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