primer3_core: 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/primer3_core - from package: primer3

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

Error in other languages:
primer3_core: bevel nie gevind nie
primer3_core: comandă negăsită
primer3_core: comando não encontrado
primer3_core: commande introuvable
primer3_core: command not found
primer3_core: käsku ei ole
primer3_core: không tìm thấy lệnh
primer3_core: komanda nerasta
primer3_core: Kommando nicht gefunden.
primer3_core: kommandot finns inte
primer3_core: komut yok
primer3_core: nie znaleziono polecenia
primer3_core: níor aimsíodh an t-ordú
primer3_core: no se encontró la orden
primer3_core: no s'ha trobat l'ordre
primer3_core: opdracht niet gevonden
primer3_core: parancs nem található
primer3_core: perintah tidak ditemukan
primer3_core: príkaz nenájdený
primer3_core: příkaz nenalezen
primer3_core: команда не найдена
primer3_core: командата не е открита
primer3_core: 命令找不到
primer3_core: コマンドが見つかりません