That sounds like your request isn't correct. You could have the URI wrong, or be missing a portion of the header.
Here's a demonstration using the Postman environment from https://postman.meraki.com
When I call GET {{baseUrl}}/organizations but don't include the key:value pair of "X-Cisco-Meraki-API-Key":{{X-Cisco-Meraki-API-Key}}, I'll get 404.
But if I ensure that my header includes that key:value pair (my API key), then I get a list of orgs that my API key works on.