用于铸造自定义令牌的服务帐户可能与您在iOS客户端中使用的服务帐户对应的项目不同.铸造令牌的后端节点服务器和调用signInWithCustomToken的客户端应属于同一项目.

It's likely that the service account you are using to mint the custom token corresponds to a different project than the one you are using in your iOS client. The backend node server minting the token and the client calling signInWithCustomToken should belong to the same project.