Hi @Huu,
We are sorry about the frustration caused, we are working to make it much easier to use.
As for the gateway creation, Sandbox/Live gateways both are discussed on the same article.
https://community2.funnelish.com/t/how-to-create-paypal-gateway-for-paypal-app-v2/902
There isn’t really that much difference between how the two work…
As for that issue, please PM me your order form url and let me check it.
Also, since you’re new to the platform better use PayPal Plugin instead of PayPal App v2 it’s much easier, unless you have a good reason to not use it 
Looking forward to helping you.