Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires sylius/sylius ~1.12.0 -> satisfiable by sylius/sylius[v1.12.0, ..., v1.12.19]. - sylius/sylius[v1.12.0, ..., v1.12.19] require sylius/calendar ^0.3 || ^0.4 -> found sylius/calendar[v0.3.0, v0.4.0] but it conflicts with your root composer.json require (^0.5.0).