How to connect to pluggable database using ipc in Oracle XE 18c?
&quot;if I use use ipc (sqlplus user/password@xepdb1) &quot;

That's not IPC, that's tcp.  It tries to find the entry for 'xepdb1' in the client's tnsnames.ora file. Then it tries to construct a tcp packet from the information found there, and send that packet to the specified host and port, where - hopefully - there is a listener.  Your error means that it could not find the specified entry in the client's tnsnames.ora.