How to connect SQLPlus with tnsnames.ora

Your command should be:

sqlplus user/password@HOST

with no space between the password and @HOST part.

With the space it treats the @HOST as a script to execute once you've logged in, and it tries to connect locally, which produced that TNS error. (As you don't log in the HOST isn't ever evaluated to establish if it exists, so it's effectively noise at this point).

C:\>sqlplus -l -s x/y @HOST
ERROR:
ORA-12560: TNS:protocol adapter error


SP2-0751: Unable to connect to Oracle.  Exiting SQL*Plus

With the space removed it looks for HOST as a TNS alias:

C:\>sqlplus -l -s x/y@HOST
ERROR:
ORA-12154: TNS:could not resolve the connect identifier specified


SP2-0751: Unable to connect to Oracle.  Exiting SQL*Plus

For me that still gets an error since I don't have HOST in my tnsnames.ora, but it's a different error and you can see it's at least trying to use it as a TNS alias. If you have it defined properly it will be able to connect to your database.