Please help me to buy beans in wechat Chanel

If you get a message -> Your purchase could not be completed, you need to update your payment method or add a new one. 

See also the following Apple articles:

Add money to your Apple ID balance

Add money to your Apple Account balance – Apple Support (UK)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

As the screenshot shows, contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide   -

Apple Support schedule phone call     -

Billings and Subscriptions   -

Apple on Twitter