Cannot purchase in-app item in games

Hello,

I have an Apple ID with a balance of NT$300, and I want to buy a NT$33 in-app item in games. However, every time I try, I get the error message:

“Your purchase could not be completed. For assistance, visitwww.apple.com/support/itunes/ww/. 

I have already checked that:

• My Apple ID balance is sufficient.
• The account region is Taiwan.
• My payment methods are up to date.

Could someone help me understand why I cannot complete this purchase and how to fix it?

Thank you!