adb shell error: device not found (Ubuntu)

I am complete novice on adb. When it comes to Android I could be classified as "dumb user" - trying however to get one level up, hence playing with adb:

$ ./adb shell

error: device not found

$ ./adb devices

* daemon not running. starting it now on port 5037 *

* daemon started successfully *

List of devices attached 

Looks like adb cannot see the connected device.

How to rectify the problem?

For your info:

• the device is visible and I can copy files to/from it


• I went through the steps suggested below and still no luck


• I tried connecting the phone to different USB ports - my comp has 2 USB controllers - one USB 3.0 and one USB 2.0

Steps to enable USB debugging on my device with 7.0 Nougat Android:

1. Go to "Configuration"


2. Go to "About Telephone / Android 7.0"


3. Click several times "Build Number" until message "You're now developer" appears


4. Go back to "Configuration" - "{ } Developer options" are now visible - go inside


5. A number of fabulous options appear - switch-on "USB Debugging"

I am using Linux and I would prefer to stay away from Windows.

Still no luck - device not found