OpenVPN 2.6 OPTIONS ERROR: failed to negotiate cipher with server. Add the server’s cipher (‘AES-256-CBC’) to –data-ciphers (currently ‘AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305’) if you want to connect to this server.

Your OpenVPN config file is an old version, for OpenVPN >=2.6 you need to change

cipher AES-256-CBC

to

data-ciphers AES-256-CBC

in your client.conf

If this is not possible, you can also try with the command line option --compat-mode 2.3.0