weechat: 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/weechat - from package: weechat

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

On Debian possible default full paths are:

/usr/bin/weechat - from package: weechat-curses

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

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