It’s not a feature I’ve used, but just to check something: your first code sample says /project/ and your return value says /projects/ (with an ‘s’). Did you try both or does the error not match the request?
$ curl -X POST https://api.glitch.com/v1/projects/achieved-creative-paneer/remix
{"message":"project id is not a valid uuid","code":"VALIDATION_ERROR"}
You may also need to get the ID of the project instead of the name.
key consideration here if you’re really doing this programmatically: you need to be authenticated, with an authorization header that contains a user’s persistent token. if the user is registered, then you’ll be fine. but if it’s an anonymous user, it’ll need some additional field in the request body with a recaptcha challenge.