discord-api-docs | Official Discord API Documentation | REST library

You're making your request to the wrong url. The Code flow is created to prevent leaking the client token. The flow is as the following

1. The Client requests a code from the discord server
2. The Client sends that code to your server
3. Your server passes the code to the discord server with it's secrets
4. Discord Server responds back to the configured redirect_uri, to obtain the token.

Note: there are 2 redirect uri, 1, which resolves the code and another which resolves the token.

You're trying to make your request directly to the token endpoint. You need to make a request to the Authorize endpoint. To obtain a token

https://discordapp.com/api/oauth2/authorize?response_type=code&client_id=157730590492196864&scope=identify%20guilds.join&state=15773059ghq9183habn&redirect_uri=https%3A%2F%2Fnicememe.website&prompt=consent