tree2hists: 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/tree2hists - from package: rootplot

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

Error in other languages:
tree2hists: bevel nie gevind nie
tree2hists: comandă negăsită
tree2hists: comando não encontrado
tree2hists: commande introuvable
tree2hists: command not found
tree2hists: käsku ei ole
tree2hists: không tìm thấy lệnh
tree2hists: komanda nerasta
tree2hists: Kommando nicht gefunden.
tree2hists: kommandot finns inte
tree2hists: komut yok
tree2hists: nie znaleziono polecenia
tree2hists: níor aimsíodh an t-ordú
tree2hists: no se encontró la orden
tree2hists: no s'ha trobat l'ordre
tree2hists: opdracht niet gevonden
tree2hists: parancs nem található
tree2hists: perintah tidak ditemukan
tree2hists: príkaz nenájdený
tree2hists: příkaz nenalezen
tree2hists: команда не найдена
tree2hists: командата не е открита
tree2hists: 命令找不到
tree2hists: コマンドが見つかりません