Hello DanielFang,
If you're unable to complete purchases in the App Store, first check to see that there is an active payment saved to your Apple ID account.
If you need to change or update your Apple ID payment method
If you've verified the payment is good, try making the purchase on another network. In the image you attached with your post, it looks like the device is connected to a Wi-Fi network. Try turning that off and using the cellular data connection to see if the purchase will go through.
Also, if you have any sort of VPN software enabled on the device turn it off.
If you're still unable to complete purchases, you'll want to contact Apple for assistance with your account: Get Support
Cheers!