To login to database via a remote connection as sysdba you will need to setup the .orapw<instance_name> file using the orapwd utility on the database server allowing remote sysdba logins with a sys password.
This is true of any remote client login wrt sysdba, not just sqldev.
Question to you: Why are you trying to login to the database as sysdba from a remote client?
ssh to the db server and do what you need to do as the super user sys.
If you do not really require to be logged in as sysdba, then create a suser with less privilages and administer the db using that user.