orangepi 4 build fail, ERROR in function prepare_host

[HeBea]

I use ubuntu18.04 virt machine to build image, but it show an error:

[ o.k. ] Preparing [ host ] [ o.k. ] Build host OS release [ bionic ] [ error ] ERROR in function prepare_host [ ./build.sh:305 -> scripts/main.sh:630 -> scripts/main.sh:435 -> scripts/general.sh:1467 -> scripts/general.sh:0 ] [ error ] It seems you ignore documentation and run an unsupported build system: bionic [ o.k. ] Process terminated

[HeBea]

why the user guide suggest that we should build it on Ubuntu 18.04 but currently build system only support ubuntu 21.04.x host?

user guide shows: Linux SDK 的编译都是在安装有 Ubuntu 18.04 的 PC 或者虚拟机（VirtualBox 或VMware）上进行的，

the general.sh script shows: Ubuntu 21.04.x (Hirsute) x86_64 is the only fully supported host OS release