mergecap: 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/sbin/mergecap - from package: wireshark

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

On Debian possible default full paths are:

/usr/bin/mergecap - from package: wireshark-common

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

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