Error when attempting to get an access token using refresh token

It looks like you're attempting to use a refresh token as a "Bearer" token in a few places in this case (where you set the "Authorization" header to "Bearer" + refreshToken). Refresh tokens themselves are not access tokens and cannot be used as Bearer tokens though, so please update your code to remove that. Refresh tokens should only be used as the "refresh_token" parameter value (as you do have) when calling /oauth2/token.

Also, I see in one line that you have a reference to "api.dropboxapi.com/oauth2/token_access_type=offline". That is not correct, and appears to be a combination of "api.dropboxapi.com/oauth2/token", which is the Dropbox OAuth 2 token endpoint, and "token_access_type=offline" which is only a parameter/option on www.dropbox.com/oauth2/authorize. You can find more information in the OAuth Guide and authorization documentation.

This comment with a basic example of the whole procedure may be a useful reference.