Type "what adb" in the terminal and see the location of adb. If the location / path to adb is not displayed, you should add the location of the platform tools folder from the sdk android to the system PATH variable.
export PATH=$PATH:/home/path_to_android_sdk/platform-tools
Alternatively, to make the change permanent, you can edit the .bashrc file and add the line above at the end. Save and run .bashrc.