rcsdiff: 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/rcsdiff - from package: rcs

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

On Debian possible default full paths are:

/usr/bin/rcsdiff - from package: rcs

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

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