pg_basebackup: could not connect to server: FATAL: no pg_hba.conf entry for replication connection from host "172.28.4.194", user "postgres", SSL off
pg_basebackup: removing contents of data directory "/backup/standby"
[postgres@de284194 ~]$
Just follow the instructions in the message: you need to add a line to pg_hba.conf. This is the file that controls the accesses to the database. It may be located in your data directory (/backup/standby) or in other places, depending on your system (by default, Ubuntu places this file by default in /etc/postgresql/...).
If you need more support, remember to state the OS and PG version.