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Tryng to install (and after updating composer):

composer create-project symfony-cmf/sandbox cmf-sandbox

I have those errors:

Installing symfony-cmf/sandbox (2.0.0)

  - Installing symfony-cmf/sandbox (2.0.0): Loading from cache

Created project in cmf-sandbox

Loading composer repositories with package information

Installing dependencies (including require-dev) from lock file

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



  Problem 1

    - Installation request for phpcr/phpcr-utils 1.3.0 -> satisfiable by phpcr/phpcr-utils[1.3.0].

    - phpcr/phpcr-utils 1.3.0 requires php ^5.6|7.0.x|7.1.x -> your PHP version (7.3.0) does not satisfy that requirement.

  Problem 2

    - phpcr/phpcr-utils 1.3.0 requires php ^5.6|7.0.x|7.1.x -> your PHP version (7.3.0) does not satisfy that requirement.

    - doctrine/phpcr-odm 1.4.2 requires phpcr/phpcr-utils ^1.2.8 -> satisfiable by phpcr/phpcr-utils[1.3.0].

    - Installation request for doctrine/phpcr-odm 1.4.2 -> satisfiable by doctrine/phpcr-odm[1.4.2].

Edit: Continuing the recommendation of Timo, I was able to install the project by executing the following:

composer create-project --ignore-platform-reqs symfony-cmf/sandbox cmf-sandbox

but I have obtained the following errors:

PHP Fatal error:  Uncaught SymfonyComponentDebugExceptionContextErrorException: Warning: count(): Parameter must be an array or an object that implements Countable in /srv/http/symfony/cmf-sandbox/vendor/symfony-cmf/routing/src/ChainRouter.php:101

Stack trace:

#0 /srv/http/symfony/cmf-sandbox/vendor/symfony-cmf/routing/src/ChainRouter.php(308): SymfonyCmfComponentRoutingChainRouter->all()

#1 /srv/http/symfony/cmf-sandbox/var/cache/dev/appDevDebugProjectContainer.php(1670): SymfonyCmfComponentRoutingChainRouter->setContext(Object(SymfonyComponentRoutingRequestContext))

#2 /srv/http/symfony/cmf-sandbox/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Container.php(335): appDevDebugProjectContainer->getCmfRouting_RouterService()

#3 /srv/http/symfony/cmf-sandbox/var/cache/dev/appDevDebugProjectContainer.php(4955): SymfonyComponentDependencyInjectionContainer->get('cmf_routing.rou...')

#4 /srv/http/symfony/cmf-sandbox/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Container.php(33 in /srv/http/symfony/cmf-sandbox/vendor/symfony-cmf/routing/src/ChainRouter.php on line 101

The project uses a Symfony version 3 structure and may have some incompatibility with PHP 7.3 (as Timo says).

You can tell Composer to ignore the PHP version requirements by adding --ignore-platform-reqs to the composer create-project command.

Else, you'll probably have to wait until the dependencies of symfony-cmf/sandbox get bumped to a later version.

Edit: If the latest release of Symfony CMF Sandbox doesn't work yet, you may try the master (latest unreleased changes) to see if they've been addressed there yet. You can do this by adding --stability=dev to your composer create-project command and using symfony-cmf/sandbox:dev-master as source instead:
composer create-project --ignore-platform-reqs --stability=dev symfony-cmf/sandbox:dev-master cmf-sandbox