Upgrading from 8.3.7 to 8.4 conflicts with acquia/blt-project
The problem was a line in my composer.json file which is prevent the upgrade from happening.

In the "conflict" section, there's a line "drupal/core": "&gt;=8.4.0-alpha1"
I have to remove that line and also run the following commands to complete the upgrade.

composer update --lock
composer require acquia/lightning:^3.0 acquia/blt:^8.9 --no-update
composer update acquia/lightning --with-all-dependencies