If you are presented with a blank screen or you find the simulator is missing in MakeCode, you may need to configure your firewall to allow specific websites. Here is a list of requirements for each editor and tool.


MakeCode

pxt.microbit.org

makecode.microbit.org

makecode.com
www.pxt.io
trg-microbit.userpxt.io
pxt.azureedge.net
github.com


Python Editor

microbit.org and subdomains (*.microbit.org)

readthedocs.io and subdomains(*.readthedocs.io)

readthedocs.org and subdomains(*.readthedocs.org)

classroom

as above and including

 *.pusher.com (enable web socket connectoins if possible)

a4fwq8ycl4.execute-api.eu-west-2.amazonaws.com

micro:bit classroom will use WebRTC if possible


Scratch 

If you are using a proxy server within a school environment, you may need to exclude the scratch link URL and port from using the proxy http://device-manager.scratch.mit.edu:20110/ 


This is set on the client and not the firewall or proxy.


How to set up an exclusion depends on how the machine's proxy settings have been configured in the first place.  This is often done through Windows Group Policy, but may instead be done through a WPAD or PAC file or configured manually on each workstation. (Source: Opendium)