By default, the Matomo (Piwik) login screen will not load in iframe, as a security measure to prevent clickjacking. You can manually enable Matomo login screen to work from inside an iframe by adding the following config file setting in your config/config.ini.php:
[General] enable_framed_pages = 1
Instead of loading the login screen in an iframe, you might want to use the logme() method to automatically log your users in Matomo (Piwik) remotely. To include any Matomo report or widget via an iframe, click to see this FAQ.