wikipedia: 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/wikipedia - from package: libwww-wikipedia-perl

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

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