Connecting Pixel 3a to Macbook gets the popup saying:

   Can't access device storage
   Make sure your device is unlocked and "File Transfer"
   or "MTP" mode is selected.
                                         [OK]

**Settings -> About Phone -> Build Number
**	(tap Build Number 7 times)


2021-6-12:

Settings -> _ -> USB Preferences
  Turn on "File transfer / Android Auto"