You will only see the option to use Alipay if you are using a Chinese IP address. Therefore you must located in China and disconnected from any other VPN or proxies. To use Alipay or Union Pay select "other methods" as shown below.
What you see next will depend on what country you are connect from (or where the VPN/Proxy you are connected to is located). If you are connect to a USA IP address you will see this.
If you are connected using a Chinese IP address (in China with VPN and proxy turned off) then you will see this.