Ensure you’re using the original cable for data transfer. Some USB cables will not work correctly for data transfer.
Afterwards install the necessary packages as indicated below.
Access/Copy Android Files
sudo apt install mtp-tools android-tools-adb
Plugin your device and it should appear in File Manager as an external drive. If this doesn’t work right away restart your system and restart your phone before and try again.
Reference:
https://superuser.com/questions/1210851/usb-file-transfer-from-linux-mint-18-1-to-android-6-device