ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

Error:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

tnsping is working

tnsping igt
TNS Ping Utility for Linux: Version 19.0.0.0.0 - Production on 16-AUG-2023 12:46:17
Copyright (c) 1997, 2020, Oracle.  All rights reserved.
Used parameter files:
/software/oracle/1910/network/admin/sqlnet.ora

Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = 10.10.90.98)(PORT = 1521))) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = igt)))
OK (0 msec)

but sqlplus gives error:
oracle@qarec-1-aps01:/software/oracle/1910>% sqlplus a/a@igt
SQL*Plus: Release 19.0.0.0.0 - Production on Wed Aug 16 12:46:33 2023
Version 19.10.2.0.0
Copyright (c) 1982, 2020, Oracle.  All rights reserved.
ERROR:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

What to check
/software/oracle/122/network/admin/listener.ora

/software/oracle/122/network/admin/tnsnames.ora
/etc/hosts

lsnrctl status

Many things can be defined wrongly...
hostname can be configured to a wrong IP, port 1521 can be closed, and more...

telnet qarec-1-aps01 1521

Trying 10.10.90.88...

telnet: connect to address 10.10.90.88: No route to host

lsnrctl status

LSNRCTL for Linux: Version 19.0.0.0.0 - Production on 16-AUG-2023 12:47:58
Copyright (c) 1991, 2020, Oracle.  All rights reserved.
Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
TNS-12543: TNS:destination host unreachable
 TNS-12560: TNS:protocol adapter error
  TNS-00513: Destination host unreachable
   Linux Error: 113: No route to host


In this case

/etc/hosts had a wrong IP for server qarec-1-aps01

listener.ora was configured with HOST = qarec-1-aps01, but tnsnames.ora with HOST = 10.10.90.98.

tnsping worked, it was using tnsnames.ora with a correct IP

sqlplus failed, it was using listener.ora with HOST = qarec-1-aps01, which got translated to a wrong IP address in /etc/hosts

ORA-00257: Archiver error. Connect AS SYSDBA only until resolved.

ORA-00257: Archiver error. Connect AS SYSDBA only until resolved.
df -hP

Archive destination /oracle_db/db2 is 100% full

Need to delete some archive files + run RMAN backup

Once /oracle_db/db2 has space, issue 

ALTER SYSTEM SWITCH LOGFILE;

/dev/mapper/OraVG02-OraVol03   200G  25M  200G   100% /oracle_db/db2

To delete old archive files:

rman target /

DELETE OBSOLETE;

or

DELETE FORCE COPY OF ARCHIVELOG ALL;

once there is space, issue

ALTER SYSTEM SWITCH LOGFILE;

/dev/mapper/OraVG02-OraVol03   200G   33M  200G   1% /oracle_db/db2