When you try to download a package from conancenter using conan download, the right syntax to use is:
conan download libcurl/8.1.1 -r conancenter
If you forget to specify -r conancenter, you will see the following error message:
usage: conan download [-h] [-v [V]] [--only-recipe] [-p PACKAGE_QUERY] -r REMOTE reference conan download: error: the following arguments are required: -r/--remote ERROR: Exiting with code: 2