1) If you try with android little icon, first open the project and then connect the cable. Also, the USB debugging in each case must be enable.
2) There is and an other way:
a) Copy and paste your .apk file in platfom-tools.
b)Connect your android device at your PC.
c)Then,open the CMD and write the path where the .apk file is located.
For example:C:\Program Files\Android\Android Studio\bin\platform-tools>
Then write this C:\Program Files\Android\Android Studio\bin\platform-tools>adb install -r nameofapk_file.apk and wait for the installation.