Replication streaming issue

Hello,

I’ve got a strange issue.
Here is the error in pg log:ERROR:  requested WAL segment 000000020000A01A0000004F has already been removed.
Everything is working fine: the lag is ok.
The check queries:
SELECT  client_addr,
        state,
        write_lag,
        flush_lag,
        replay_lag 
FROM    pg_stat_replication 
WHERE   application_name ='walreceiver’;

SELECT COALESCE(ROUND(EXTRACT(epoch FROM now() - pg_last_xact_replay_timestamp())),0) AS seconds;

How could I monitor the problem? Why the replication streaming is still working?

Thank you in advance.
Mai