tls_prune: command not found

Have You tried accessing the binary with the full path?

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/lib/cyrus/bin/tls_prune - from package: cyrus-common

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