Are you using the correct username and password specifically for openvpn and not the prontonvpn account?
How to manually set up port forwarding
https://protonvpn.com/support/port-forwarding-manual-setup/#:~:text=To use port forwarding%2C add,suffixes supported by Proton VPN.
How to log in to Proton VPN
https://protonvpn.com/support/vpn-login/