Python 3.7 API upload error

This error message indicates that the access token you're using isn't of the expected format. You appear to be storing your access token in that "dropbox_access_token" field. To be clear though, you've redacted it for this forum post, correct? (If not, note that attempting to just uses the 'x's will fail. You need to replace it with an actual access token from Dropbox.)

Can you double check that the access token string you have saved there is the exact value you received from Dropbox when initially retrieving the access token? It will generally be a 64 character string of letters, numbers, and - or _.  Make sure there aren't any extra characters accidentally added on, for instance.

Also, how are you loading that string into the Dropbox 'client' object? Please make sure it isn't accidentally getting modified somewhere along the way.

Another thing you can do is to add some extra logging of the network call, so you can inspect what is actually getting sent:

import http.client as http_client
http_client.HTTPConnection.debuglevel = 1

(Make sure you don't post your access token here.)