Did you turn on usb debugging? This is required for adb. USB debugging is found in the Developer Options setting.
I also use adb mostly to transfer files.
Once you enable usb debugging, you can check if adb is attached to your device by typing the code below on the terminal.
Note: On some android versions, you have to confirm the connection by clicking YES or OK.
You will then see something like this. The last line will be different from mine of course.
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
List of devices attached
LGLS770eba12ac3 device
In my example, LGLS770eba12ac3 is my LG android phone. Now that it is attached, I can use adb to interact with the device.
If you get no devices found, then USB debugging is off or some other reason.