Unable to update access token in postman.

I see you're trying to call the /2/auth/token/from_oauth1 endpoint. If this is a new integration, you should not be using that endpoint. This endpoint is only meant for use if you have pre-existing OAuth 1 access tokens, and the oauth1_token and oauth1_token_secret parameters would refer to the old OAuth1 access token key and secret, not your client ID and secret. This endpoint should not be used as part of the current OAuth 2 app authorization flow.
 
OAuth 1 access tokens were a feature of an old version of the Dropbox API. If you've only used API v2 and OAuth 2, you wouldn't have any OAuth 1 access tokens. If you don't have pre-existing OAuth 1 access tokens, for instance, if you're just developing the integration now, you should send the user through the normal OAuth 2 app authorization flow.
 
You can find information on how this works in the OAuth Guide and authorization documentation.