Did you read the documentation?
Take a look at point 2:
(…) Test your client configuration: Run the client from a CLI prompt in the ‘vpn’ folder, preferably as root. For example:
sudo openvpn --cd /rw/config/vpn --config openvpn-client.ovpn
Watch for status messages that indicate whether the connection is successful and test from another VPN VM terminal window with ping. (…)
The whole procedure is explained in detail, I don’t think it can be more clearly.
I don’t get your fixation with the secure-core servers. Yes, they do work, like every other ovpn files I tested. Like I said before. Did you ever take a look at the files? Everything is explained in there, also the additional options like special suffixes to your OpenVPN username for selecting the exit IP corresponding to a specific server, ad blocking and anti-malware filtering etc.
You don’t need all this but you could at least take a look. If you compared your beloved secure-core files to any server config you’d see that this is not so special.
Sorry, I don’t think I can help you any further because I am wondering if you even are a paid subscriber of protonvpn.
It is possible that a certain server is down or at 100% so you should at least try another and just see if it connects before doing all the other stuff.
There are people trying wrong username & password (no, it’s not the login-data for your account) and other possibilities but it could be anything, so I say good luck.