The error message "Can't access device storage, Make sure that your device is unlocked and 'File Transfer' or 'MTP' mode is selected" can be solved using the following steps (on most Android phones):
1. Go into your Android phone settings (gear icon).
2. Try "Storage and USB" but if you can't find "MTP (Media Transfer Protocol) and enable it, you need to:
3. Under "About Phone" go to "Build Number" at the bottom and tap it 7 times, this enables Developer Mode.
4. Back under Settings, you will now have a "Developer Options" category, and within this massive listing you should be able to find "Select USB Configuration", under which you may already have MTP enabled. I had to toggle it to "Charging" then back to "MTP Media Transfer Protocol" in order to have my Mac recognize the change and actually display the directory under Android File Transfer.
This gave me access to the files on my android phone from within Android File Transfer.
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