user login api

There isn’t one. At least not built-in. You’ll need to use one of the plugins. Which one depends on which method you want to use.

See the Authentication section of the documentation: https://developer.www.ads-software.com/rest-api/using-the-rest-api/authentication/

i have already added rest-api wp plugin too. register is working but only login option ! not working…

You don’t need the REST API plugin, that plugin is a remnant from before the API was included in core code. What you do need is some sort of authentication plugin as described in Jacob’s earlier link. The default cookie authentication is unsuitable for mobile apps.