g_covar: 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/g_covar - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_covar - from package: gromacs

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

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