sizeseq: 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/sizeseq - from package: emboss

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

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