rpmbuild -tb openvpn-2.5.2.tar.gz get read spec file failed

mbj2021

I want build from source code for rpm, command is:
rpmbuild -tb openvpn-2.5.2.tar.gz

and get a error:
from openvpn-2.5.2.tar.gz read spec file failed

The source code is download from openvpn.net.

How can I fixed it?