Mounting a CIFS share using Linux may result in the following error

mount error(95): Operation not supported

This often relates to a failure to negotiate an authentication level between the server and the client (usually because Ubuntu and derivatives default to SMBv1, however this has been fully deprecated and all current systems should be using SMBv3)

Test by querying the server using smbclient

smbclient -o username={username} //f{fqdn}/{share_path} -U {user}

If you receive and output similar to below, then connect using a manually defined version

protocol negotiation failed: NT_STATUS_INVALID_NETWORK_RESPONSE

smbclient -o username={username},vers=3.0 //{fqdn}/{share_path} -U {user}