Why am I getting a "You are not authorized to access this page." on the Drupal Migrate UI page?
Are you logged in? Is your login the superuser? You can always get into your local site w/ drush:

$ cd /path/to/docroot
$ ../vendor/bin/drush uli --no-browser
http://default/user/reset/1/1527085618/[RANDOM_STRING]/login


Assuming your setup uses the drupal-project composer approach. Otherwise, you could use drush w/ a site alias or params.

If you're logged in as the superuser and still have issues, that's indicative of something wrong w/ the routing specification. That shouldn't happen with a contrib module, unless you've made any modifications? If you haven't and the superuser can't access the UI, then I would file an issue w/ the migrate module.