htmldiff: 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/htmldiff - from package: rubygem-diff-lcs
/usr/lib/ruby/gems/1.8/gems/diff-lcs-1.1.2/bin/htmldiff - from package: rubygem-diff-lcs

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

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