pg_comparator: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/pg_comparator - from package: postgresql-comparator
/usr/lib/postgresql/9.4/bin/pg_comparator - from package: postgresql-comparator

You may install the required package with command apt-get install {package}

Error in other languages:
pg_comparator: bevel nie gevind nie
pg_comparator: comandă negăsită
pg_comparator: comando não encontrado
pg_comparator: commande introuvable
pg_comparator: command not found
pg_comparator: käsku ei ole
pg_comparator: không tìm thấy lệnh
pg_comparator: komanda nerasta
pg_comparator: Kommando nicht gefunden.
pg_comparator: kommandot finns inte
pg_comparator: komut yok
pg_comparator: nie znaleziono polecenia
pg_comparator: níor aimsíodh an t-ordú
pg_comparator: no se encontró la orden
pg_comparator: no s'ha trobat l'ordre
pg_comparator: opdracht niet gevonden
pg_comparator: parancs nem található
pg_comparator: perintah tidak ditemukan
pg_comparator: príkaz nenájdený
pg_comparator: příkaz nenalezen
pg_comparator: команда не найдена
pg_comparator: командата не е открита
pg_comparator: 命令找不到
pg_comparator: コマンドが見つかりません