It was solved.
As matedata.yaml said,“This charm was written using v3.0.1, so earlier versions of EasyRSA may
not work. You can find the releases of EasyRSA at
https://github.com/OpenVPN/easy-rsa/releases”
wget https://github.com/OpenVPN/easy-rsa/releases/download/3.0.1/EasyRSA-3.0.1.tgz
juju attach easyrsa easyrsa=./EasyRSA-3.0.1.tgz