seqtk: 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/seqtk - from package: samtools

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

On Debian possible default full paths are:

/usr/bin/seqtk - from package: seqtk

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

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