from your compile message, seems you didn’t make it correct. I am using Openwrt buildroot, not SDK. So it may be a little different.

First, you don’t need to git clone the badvpn.

create a folder, openwrt/packages/badvpn,

then put the Makefile I attached in this folder. Run make menuconfig and select badvpn in network->vpn, then

make package/badvpn/compile