Wondering why my Pi VPN server (PiVPN) can only use TCP protocol but not UDP protocol [closed]

I have to make a personal VPN server using PiVPN with my Raspberry Pi 3B+ (Raspbian Jessie). I make my server according to this guide: https://www.cloudpro.co.uk/it-infrastructure/virtualization/7503/how-to-turn-a-raspberry-pi-into-a-vpn-server I follow the instruction until I met a problem. The guide says that I have to choose UDP (port 1194) to configure my server, port-forwarding the same port with…