fldigi: 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/fldigi - from package: fldigi

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

On Debian possible default full paths are:

/usr/bin/fldigi - from package: fldigi

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

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