imp-admin-upgrade: 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/imp-admin-upgrade - from package: php-horde-imp

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

On Debian possible default full paths are:

/usr/bin/imp-admin-upgrade - from package: php-horde-imp

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

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