Your requirements could not be resolved to an installable set of packages. Problem 1 - dynamic/flexslider is present at version 5.x-dev and cannot be modified by Composer - Root composer.json requires dynamic/silverstripe-linkable ^2.0 -> satisfiable by dynamic/silverstripe-linkable[2.x-dev (alias of dev-master)]. - Root composer.json requires silverstripe/installer 5.x-dev -> satisfiable by silverstripe/installer[5.x-dev]. - dynamic/flexslider 5.x-dev conflicts with silverstripe/vendor-plugin 1.5.1. - dynamic/silverstripe-linkable dev-master requires silverstripe/vendor-plugin ^1.0 -> satisfiable by silverstripe/vendor-plugin[1.0.0, ..., 1.x-dev]. - silverstripe/installer 5.x-dev requires silverstripe/vendor-plugin 2.x-dev -> satisfiable by silverstripe/vendor-plugin[2.x-dev]. - silverstripe/vendor-plugin[1.0.0, ..., 1.4.1] require composer-plugin-api ^1.1 -> found composer-plugin-api[2.6.0] but it does not match the constraint. - dynamic/silverstripe-linkable 2.x-dev is an alias of dynamic/silverstripe-linkable dev-master and thus requires it to be installed too. - You can only install one version of a package, so only one of these can be installed: silverstripe/vendor-plugin[1.0.0, ..., 1.x-dev, 2.0.0-alpha1, ..., 2.x-dev].