psql: 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 CentOS possible default full paths are:

/usr/bin/psql - from package: postgresql

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/psql - from package: postgresql-client-common
/usr/lib/postgresql/9.4/bin/psql - from package: postgresql-client-9.4

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

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