Route "username.admin_form" does not exist.

Hey @thaoquang,
I have tried to reproduce the error by following the steps that you provided with the same version that you have given.
But I couldn't able to do so.I'm attaching the screenshot for your reference. let me know if I'm missing something.

Thanks for rep!

Well, the error show both drush and install ui after I add display error config to settings.ph

Edit: btw, the config form still working as descibe.

I've also added display config in settings.php still not getting any error. can you please attach any screenshot of error for better understanding.
Thank you

In my config

error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);

I'm still not able to reproduce this issue so postponing this issue until someone else finds the same error.
Thanks!!