Oracle RMAN-06136: ORA-17629: Cannot connect to the remote database server

Question

I am creating a standby database using rman (Oracle 18c) from my main database which is an Oracle restart that uses ASM but when I issue the command

duplicate target database for standby from active database nofilenamecheck;

I keep getting the same error :

RMAN-03002: failure of Duplicate Db command at 03/26/2019 02:22:24 RMAN-05501: aborting duplication of target database RMAN-03015: error occurred in stored script Memory Script RMAN-06136: Oracle error from auxiliary database: ORA-17629: Cannot connect to the remote database server ORA-17627: ORA-12154: TNS:could not resolve the connect identifier specified ORA-17629: Cannot connect to the remote database server

So it's clear that the error means that there is something wrong with the connection to to the standby database but here the strange part, I checked tnsnames.ora and listener.ora on both servers and everything is OK.

I tested the connection to bothe DBs from both sides using tnsping and it works just fine (A DB to B DB from A DB & A DB to A DB from A DB & B DB to A DB from B DB and B DB to B DB from B DB).

I also tested the connection using sqlplus from both sides and again it works fine.

I even tried to connect to each database from RMAN from both DBs and it always works

So what is wrong? Am I missing something?