Symfony 5 & EasyAdmin 3.0 - /admin route not found

You need to create at least one dashboard. Try:

php bin/console make:admin:dashboard

Next, you can create a CrudController with:

php bin/console make:admin:crud

Yeah, I am reading this book right now and ran into the same issue.

First, make sure that your working directory is clean (run "git status" and remove all changes made by EasyAdminBundle setup).

Then run:

composer require easycorp/easyadmin-bundle:2.*

to install EasyAdminBundle version 2; with this version, you can proceed as described in the book.