ORA-12521: TNS:listener does not currently know of instance requested, even after TWO_TASK set , but

DB 2 node RAC. 11.2.0.3 , EBS - 12.1.3

echo $TWO_TASK
UATERP

Enter user-name: apps
Enter password:
ERROR:
ORA-12521: TNS:listener does not currently know of instance requested in
connect descriptor


Enter user-name: apps@UATERP
Enter password:

Connected to:              ========== GOES WELL.


Enter user-name: apps
Enter password:
ERROR:
ORA-12514: TNS:listener does not currently know of service requested in connect
descriptor

Enter user-name: apps
Enter password:

Connected to:

Getting Random behavior on the connection. sometimes connects some time not.

tnsping:  Attempting to contact (DESCRIPTION= (ADDRESS=(PROTOCOL=tcp)(HOST=hqebsuat)(PORT=1541)) (CONNECT_DATA= (SERVICE_NAME=UATERP) (INSTANCE_NAME=UATERP1)))
OK (10 msec)
hqebsuat - is SCAN Hostname for RAC db node. 1541 is scan port.