Error when creating CA for OpenVPN server

I am following the guide at OpenWRT to create at OpenVPN from the official web: OpenVPN (Streamlined)

But I got the following error at the very first steps of the guide, when generating the CA introducing the following command:

openssl req -x509 -new -sha512 -days 3650 -newkey rsa:4096 -keyout ca/OpenWrt-CA.key.pem -out ca/OpenWrt-CA.crt.pem -config ./openssl.cnf -extensions v3_ca_openwrt

I am getting the following output, but I do not know where the error is and how to solve it (I guess it is something related with the openssl.conf file):

3069531332:error:0E079065:lib(14):func(121):reason(101):NA:0:line 1

Thanks in advance.