Composer install dependency error drupal/drupal dev-master
I ran into the same issue. My issue was related to upgrading to a new Drupal core minor version (ex: 8.6 to 8.7). If that's not the case for you, then my steps may be unnecessary.

I was able to resolve it by removing the core directory of Drupal. Then run composer update drupal/core --with-dependencies. I believe the issue stems from the core/composer.lock file having conflicts with what you are trying to install.