If you're working directly with an .ovpn file (as is often supplied by a paid VPN service), within that file you need the line:
auth-user-pass /etc/openvpn/credentials.auth
This is on one line.
If you were starting OpenVPN manually from the command line, auth-user-pass alone will prompt you to type your username and password every time. When starting OpenVPN automatically as a service that is not possible, so you need to modify the configuration to use a user-pass file.
When running from an .ovpn file, the only LuCI option you can set is to point configuration to that file. Make all configuration settings by editing the .ovpn file. Do not mix UCI / LuCI settings with a .ovpn file.