You go to check the inbox at the location where you host the email address which you entered as the admin email during the installation of WordPress.
But judging by your question, I suspect you don’t know the email address, didn’t enter a working email address or entered an email address which you cannot create email addresses on. Or all of the above.
What were you doing just before you saw this message? Were you installing or updating a particular plugin? If so, the plugin installation is likely corrupted, which means you need to disable it to regain access. You can do that by removing the plugin files through a file manager.
If you were updating WordPress, then the installation may be corrupted. If so, you can follow these instructions to fix it: