In some cases, the computers using MCreator are behind a firewall or proxy. A common case of this is classrooms in schools, summer camps, and STEM workshops.
In order to be able to use MCreator under a restrictive firewall, at least the following domains need to be whitelisted:
If protocol selection is required, allow both HTTP and HTTPS connections.
If the classroom does not have an internet connection, you can use the offline mode of MCreator that can be enabled in preferences. Keep in mind that the initial setup and first workspace creation for a given Minecraft version still require internet connection.
Whenever possible, avoid using offline mode as it can cause many unpredictable errors and cache corruption which requires intervention described on this page: https://mcreator.net/wiki/gradle-setup-errors.