How to get a refresh token

If you're using a long-lived access token (e.g., as you described, by setting the "Access token expiration" setting in the App Console to "No Expiration" and then using the "Generate" button to generate an access token for your account and app), then you don't also need a refresh token. (Refresh tokens are for use with short-lived access tokens, not long-lived access tokens. Please note that long-lived access tokens are considered deprecated though. )

The particular error you're getting here is indicating that you did not pass a non-empty value for either the oauth2_access_token parameter (for a long-lived or short-lived access token) or oauth2_refresh_token (for a refresh token) when building your client. At least one of these parameters is needed to make API calls.

How are you passing your access token in and constructing that Dropbox object? It sounds like something about your environment isn't successfully retrieving your access token when running from the cron job.