io.so: command not found

Have You tried accessing the binary with the full path?

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/pkg/io/bin/io.so - from package: gap-io

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