According to this documentation, the request requires a JSON body containing a recaptchaToken, iosSecret, iosReceipt, and phoneNumber, are you sending those values?
That being said, I think you are using an older URL, you should be using the latest Identity Toolkit API located at https://identitytoolkit.googleapis.com instead:
https://cloud.google.com/identity-platform/docs/reference/rest
https://cloud.google.com/blog/products/identity-security/getting-started-with-identity-platform
https://developers.google.com/identity/toolkit/migrate-identityplatform