As noticed by Visvanath in this forum post: https://moodle.org/mod/forum/discuss.php?d=269024 and by myself when trying to register the School demo as a test.
It seems the site is registering fine but this error message appears.
Initially I checked to see if the site was already registered. It was, so I deleted the original registration and tried to re-register. The error message appeared but the site did re-register correctly.