The error occurs because the auto-setup script created by QtCreator (build/.qtc/package-manager/conan_provider.cmake) cannot determine the current C++ standard. Take a look at the function function(detect_host_profile output_file). The error is fixed by moving the C++ standard setting to the very beginning of your CMakeLists.txt script, before the project keyword. For example, the correct CMakeLists.txt: cmake_minimum_required(VERSION 3.16) # it is important that the standard be defined before the project keyword set(CMAKE_CXX_STANDARD 20) set(CMAKE_CXX_STANDARD_REQUIRED ON) project(i18n_strict LANGUAGES CXX)