dbtoepub: 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/share/sgml/docbook/xsl-ns-stylesheets-1.75.2/epub/bin/dbtoepub - from package: docbook5-style-xsl
/usr/share/sgml/docbook/xsl-stylesheets-1.75.2/epub/bin/dbtoepub - from package: docbook-style-xsl

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

On Debian possible default full paths are:

/usr/bin/dbtoepub - from package: dbtoepub

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

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