How to buy fireworks for Fantasy Westward Journey? Explain what always says I'm wrong

There are two options to buy fireworks, 1 is "I am interested", and this can only be purchased separately. You can't add commas. I think this is what you ordered.

The second one is "I want to buy a serial number". For example, buy 20 numbers from 2 1 to 40, and enter 20,40.

There is no comma after the second number 40. And you can only buy 30 numbers at a time. The second number is greater than 1.