rational.o: 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/lib/x86_64-linux-gnu/gap/bin/rational.o - from package: gap-dev

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

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