Perhaps your executable is in some folder that that has been left out of your $PATH and so the system is unable to find the program when you try to run the program
will print out the $PATH variable that tells where executables are located
is your executable in one of these folders?
if you need to append new folders to PATH you can run
but change will end when you end the session you typed it into, for a more permanent solution add the above command to either your /etc/profile or /home/.bashrc or your preferred initialization script
better than appending endless strings of directories onto your path it would be best to collect all executables into a single folder and add the one folder to your path:
# cp ./executable /home/USER/commands/
# echo "PATH=$PATH:/home/USER/commands">>/home/.bashrc