neqn: 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/neqn - from package: groff

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

On Debian possible default full paths are:

/usr/bin/neqn - from package: groff-base

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

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