I am stuck trying to obtain a token via the JWT Bearer Token Flow. I followed the steps here https://dreamevent.secure.force.com/articleView?id=remoteaccess_oauth_jwt_flow.htm&type=5

1. Created the connected app and generated a JWT Bearer Token using the sample code provided by Salesforce

2. Authorised the app via the browser successfully with a POST request like the below

https://test.salesforce.com/services/oauth2/authorize?response_type=code&client_id=3MVG9KlmwBKoC7U3w7pS2H7mWZgOv7hyIA9Tj...90JmPpk&redirect_uri=https://test.salesforce.com/services/oauth2/success

3. Now trying to execute this from Postman using the below

https://test.salesforce.com/services/oauth2/token?assertion=e...ip5reUNICJbwa_tEDErguFZuSVjv8_eaO_wM6D9cM6ycAA73YBcw&grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer

 I keep on receiving the error 
    "error": "invalid_grant",
    "error_description": "expired authorization code"

Anybody experienced this previously? I would be grateful for help on this