Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires psalm/plugin-mockery ^0.9.1 -> satisfiable by psalm/plugin-mockery[0.9.1]. - psalm/plugin-mockery 0.9.1 requires vimeo/psalm ^4.5 -> found vimeo/psalm[4.5.0, ..., 4.x-dev] but it conflicts with your root composer.json require (~5.15.0).