Thanks for reaching out to the Zoom Developer Forum and welcome to our community!
By taking a look at the code snippet you shared with us, it looks like you are passing the wrong grant_type.
To request an access token using Server to Server OAuth credentials, the grant_type in the request must be “account_credentials” , this could be the reason why your request to retrieve meetings is failing.
Also, here is a post on How to user Server to Server Oauth app with Postman, that also walks you through the settings needed to be able to use this app type:
Let me know if this helps,
Elisa