Fix - RHEL/CentOS 6 is not supported by DirectAdmin anymore - Cannot Execute Your Request /usr/local/bin/lego is required, exiting...

I recommend updating your OS, but you can still install lego in the following way:
Open : /usr/local/directadmin/custombuild/build
Edit : remove "exit 1" 

elif [ "${OS_CENTOS_VER}" = "6" ]; then

echo "RHEL/CentOS ${OS_CENTOS_VER} is not supported by DirectAdmin anymore."

#exit 1

fi
 

And then run build lego:

cd /usr/local/directadmin/custombuild
./build letsencrypt
./build lego
systemctl restart directadmin