Aurora RDS async replication to historical MySQL cluster

I was trying to do https://dev.mysql.com/doc/refman/8.0/en/replication-howto-masterstatus.html until the command to pause all writes gets access denied on my test db FLUSH TABLES WITH READ LOCK; Error Code 1045. Access denied for user 'admin'@'%' So in Aurora RDS, how do I pause all writes, grab the binary log filename and offset, take snapshot, and then unpause all writes? Then, I can restore the snapshot, and I hope setup to start replicating from that offset to catch up to the source database.