Get refresh token from access token?

It's not possible to get a refresh token from an access token. A refresh token can only be retrieved by authorizing the app via the OAuth app authorization flow. (The "Generate" button on an app's page on the App Console does not offer the ability to get a refresh token; that only returns an access token.)  To get a refresh token for a user account, an app should implement the OAuth app authorization flow, and request "offline" access. You can also find more information in the authorization documentation.

The /oauth2/token call you shared is failing because the "code" parameter there expects an "authorization code", not an access token.

If this is just for your own account though, you can process this manually without implementing the OAuth app authorization flow in your app's code/UI.

For instance, you could:

1. Make your OAuth app authorization URL like this: (plug in your app key in place of "APPKEYHERE").

https://www.dropbox.com/oauth2/authorize?client_id=APPKEYHERE&response_type=code&token_access_type=offline

2. Browse to that page in your browser while signed in to your account and click "Allow" to authorize it.

3. Copy the resulting authorization code.

4. Exchange the authorization code for an access token and refresh token like this, e.g., using curl on the command line: (plug in the authorization code from step 3 in place of "AUTHORIZATIONCODEHERE", the app key in place of "APPKEYHERE", and the app secret in place of "APPSECRETHERE").

curl https://api.dropbox.com/oauth2/token \
    -d code=AUTHORIZATIONCODEHERE \
    -d grant_type=authorization_code \
    -u APPKEYHERE:APPSECRETHERE​

The response will contain a short-lived access token and refresh token that you can then use as needed.

5. Store the returned refresh token. (It can be repeatedly re-used and doesn't expire by itself, though it can be revoked on demand.)

6. Use the returned short-lived access token to make API calls until it expires. For example, here's how a call to get the connected user information would look like: (plug in the access token from step 4 in place of "ACCESSTOKENHERE")

curl -X POST https://api.dropboxapi.com/2/users/get_current_account \
    --header "Authorization: Bearer ACCESSTOKENHERE"

7. Retrieve a new short-lived access token whenever needed like this, e.g., using curl on the command line: (plug in the refresh token from step 5 in place of "REFRESHTOKENHERE", the app key in place of "APPKEYHERE", and the app secret in place of "APPSECRETHERE")

curl https://api.dropbox.com/oauth2/token \
   -d refresh_token=REFRESHTOKENHERE \
   -d grant_type=refresh_token \
   -d client_id=APPKEYHERE \
   -d client_secret=APPSECRETHERE

8. Use the returned short-lived access token to make API calls until it expires. For example, here's how a call to get the connected user information would look like: (plug in the new access token from step 7 in place of "ACCESSTOKENHERE")

curl -X POST https://api.dropboxapi.com/2/users/get_current_account \
    --header "Authorization: Bearer ACCESSTOKENHERE"

Repeat steps 7 and 8 programmatically as needed.

Hope this helps!