Why am I getting a "You are not authorized to access this page." on the Drupal Migrate UI page? I just installed this module on my Drupal 8 site hosted locally on Acquia Dev Desktop. I haven't had this issue with any other modules and this is occurring immediately after I install the module.
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.