fitshdr: 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/fitshdr - from package: wcslib-utils

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

On Debian possible default full paths are:

/usr/bin/fitshdr - from package: wcslib-tools

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

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