fldigi-shell: 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/fldigi-shell - from package: fldigi

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

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