fmerge: 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 Debian possible default full paths are:

/usr/bin/fmerge - from package: fhist

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

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