I tried last instructions and did not work for me.
With the authorization token received in step 3 (I did the first two without issues), I tried to add the account using insync-headless but received the following message:
Sorry, an error occurred: (<HTTPStatus.BAD_REQUEST: 400>, b'{\n "error": "redirect_uri_mismatch",\n "error_description": "Bad Request"\n}')
When I reviewed my google account page I saw that Insynchq was not in the allowed applications
This means that, although I received a token, the application didn’t register properly in my google account.