ansible-doc: 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/ansible-doc - from package: ansible

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

On Debian possible default full paths are:

/usr/bin/ansible-doc - from package: ansible

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

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