Your requirements could not be resolved to an installable set of packages.

Problem 1

• The requested package ezsystems/ezpublish-kernel 5.2.0@rc could not be found.

Potential causes:

• A typo in the package name
• The package is not available in a stable-enough version according to your minimum-stability setting
  see <https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion> for more details.

The problem is in the root composer.json where it is

"ezsystems/ezpublish-kernel": "5.2.0@rc",

it should be

"ezsystems/ezpublish-kernel": "5.2.*@rc",